Instead of value being "" or omitted meaning "use group.Default", we now allow the empty value
to go through. This was done to allow multi-value options to be able to encode 'no options selected'.
This is important as the alternative is very confusing, as 'nothing selected' would generally mean
'match everything', not 'select default' (which in the k8s namespace example, means the "default" namespace).

Since the UI is the only user of this API, and it already sends the default value always, this does not affect
any existing usage.

Since the UI still wants to show a 'match all' button to prevent confusion, but it's not a normal option
(if it were a normal option, it could be combined with others, which isn't meaningful), we add a new key
NoneLabel that indicates the name that should be displayed on this extra button.

Finally, we implement these changes for the k8s namespace button, ie.
- Get rid of All Namespaces option
- Add "All Namespaces" as the NoneLabel
- Default to "" when "default" namespace not present, which is equivalent to the old "all" option.

This is the first usage of the new 'union' selectType.
Note that we're still sending 'all' for now. There's currently no easy
way to specify this meaning, and arguably it should be done entirely clientside.
But for now it just means some UI weirdness where 'all' is one of the options and having it on
means anything else you select doesn't matter.

This field changes the option group behaviour depending on its value.
Currently only supports two values:
"one" (default): Old behaviour, one option can be selected
"union": Any number of options can be selected, and the filters are OR-ed togther

It is written in such a way as to easily enable a future "intersection" option,
as per union but AND-ing the filters. But this is not done here. YAGNI.

This was causing a large number of repeated appending of the same option group

This reverts commit d55c528f.

This reverts commit 76ddc75f, reversing
changes made to 3ade2933.

We are rolling this back for now because it's causing a bug where sub-topologies
would have ~3000 repeated cases of the k8s filters, causing performance issues clientside.

To facilitate this, we replace the existing functionality of updateFilters which
sets k8s topologies to have the filters [namespace, managed], to instead append the namespace filter
to any existing. This lets it apply to both k8s and container topologies without overwriting existing
container filters. We instead set the managed filter in the static definition.

This however has the side effect that the ordering of the namespace filter and the managed filter
in k8s topologies has been reversed, so it reads:
	Show Unmanaged | Hide Unmanaged
	foo | bar | default | baz | All Namespaces
instead of:
	foo | bar | default | baz | All Namespaces
	Show Unmanaged | Hide Unmanaged

Also, refactor some tests and MakeRegistry in api_topologies

(not working yet)

Also, make name consistent with other subtopologies

gofmt load_container_filters.go

removed the environment variable for container label filters

Added the --app.container-label-filter command line argument, and load_container_filters.go now uses the results from that

Changed init() to InitializeTopologies()

Changed init() to InitializeTopologies() so that it can be called after the container filters are loaded from the command line argument. init() executes before main() in prog/main.go, so the flag parsing isn't finished before init() is called

Applied lint fixes

fixed lint issues

brought back the init function for api_topologies.go

Addressed many of the PR comments, except escaping colons

Renamed IsDesired to HasLabel in render/filters.go

Allows for the user to escape colons

added registry function for modifying the container filters

created a separate function that parses the container filter flags

simplified registry.addContainerFilters()

addressed review comments

switched API Topology Description IDs to constants

addressed review comments

joined constants

added test functions

addressed most of the review comments

Changed containerLabelFilters to an array of APItopologyOptions, placing the parsing in the Set() function. Removed parsing from HasLabel in render/filters.go

refactored code

added test that applies to the container filtering by labels

applied golint

made Registry items private and added a MakeRegistry() function

fixed usage of topologyRegistry.RendererForTopology

Added container label filters by exclusion

minor update to report_fixture

Modified container labels test to use existing report

I added labels to the existing containers in the fixed report for testing.

refactored code

refactored code

further code refactoring

addressed @ijsnellf's review comments

unexported Registry, and reduced duplicate code

addressed @ijsnellf's review comments

Addressed review comments

Addressed final review comments

They are of no interest to most users and affect the initial user
experience.

Fixes #1689.

* Add filters for pseudo nodes.

- Don't filter the internet node as a pseudo node.
- Rename pseudo filter to unmanaged/uncontained.
- Review feedback
- Move the FilterFoo funcs into the tests
- Drop the 'nodes' from filter labels.

* Fix experimental

Squash of:

* We have to keep all the container hostnames until the end so we can
  count how many we've filtered

* Adding tests for ContainerHostnameRenderer and PodServiceRenderer with
  filters

* Because we filter on image name we need the image name before
  filtering

* Alternative approach to passing decorators.

* Refactor out some of the decorator capture

* Don't memoise decorated calls to Render

* Fixing filtered counts on containers topology

  Tricky, because we need the filters to be silent sometimes (when they're
  in the middle), but not when they're at the top, so we take the "top"
  filter's stats. However, this means we have to compose all
  user-specified filters into a single Filter layer, so we can get all
  stats.

  There are no more Silent filters, as all filters are silent (unless they
  are at the top).

  Additionally, I clarified some of the filters as their usage/terminology
  was inconsistent and confused. Now Filter(IsFoo, ...) *keeps* only nodes
  where IsFoo is true.

Squash of:
- use detailed.Summaries to render topology nodes
- ban merging nodes of different topologies (they should be mapped)
- need to prune parents when mapping node types
- render container images by id if they have no name
- remove separate render ids and prune parents in NewDerived*
- don't render metrics/metadata for groups of nodes
- fixing up tests
- removing pending unit tests (for mapping.go, for now)
- updating experimental dir for RenderableNode removal