document force_pull and :latest tag interaction

fifo: Use plain fifo file in Unix

UI: Reattach resources to stats trackers in the event they were destroyed

Also, make the test use files just like in the non-test case.

This PR switches to using plain fifo files instead of golang structs
managed by containerd/fifo library.

The library main benefit is management of opening fifo files.  In Linux,
a reader `open()` request would block until a writer opens the file (and
vice-versa).  The library uses goroutines so that it's the first IO
operation that blocks.

This benefit isn't really useful for us: Given that logmon simply
streams output in a separate process, blocking of opening or first read
is effectively the same.

The library additionally makes further complications for managing state
and tracking read/write permission that seems overhead for our use,
compared to using a file directly.

Looking here, I made the following incidental changes:
* document that we do handle if fifo files are already created, as we
rely on that behavior for logmon restarts
* use type system to lock read vs write: currently, fifo library returns
`io.ReadWriteCloser` even if fifo is opened for writing only!

exec: report cpu usage summary stat

All allocation stats are routable from the server

Adding tests for killing task without grace

Co-Authored-By: notnoop <mahmood@notnoop.com>

Convert all cpu and memory usage fields regardless of stated measured
fields, and handle swap fields

Fix a case where TotalTicks doesn't get serialized across executor grpc
calls.

Here, I opted to implicit add field, rather than explicitly mark it as a
measured field, because it's a derived field and to preserve 0.8
behavior where total ticks aren't explicitly marked as a measured field.

docker: restore pre-0.9 container names

As far as I can tell Nomad itself does not use the container name after
container creation, so this should be safe.

OP: https://groups.google.com/d/topic/nomad-tool/kYkyERfVRXE/discussion
v0.8.7 code: https://github.com/hashicorp/nomad/blob/v0.8.7/client/driver/docker.go#L1530-L1531

* Divest api/ package of deps elsewhere in the nomad repo.

This will allow making api/ a module without then pulling in the
external repo, leading to a package name conflict.

This required some migration of tests to an apitests/ folder (can be
moved anywhere as it has no deps on it). It also required some
duplication of code, notably some test helpers from api/ -> apitests/
and part (but not all) of testutil/ -> api/testutil/.

Once there's more separation and an e.g. sdk/ folder those can be
removed in favor of a dep on the sdk/ folder, provided the sdk/ folder
doesn't depend on api/ or /.

* Also remove consul dep from api/ package

* Fix stupid linters

* Some restructuring

docs: mention support for centos6 is dropped

Fix json parsing bug with plugins that don't provide args

This fixes a bug with JSON agent configuration parsing where the AST
for the plugin stanza had unnecessary flattening originating from hcl parsing
library. The workaround fixes the AST by popping off the flattened element and wrapping
it in a list. The workaround comes from similar code in terraform.

There were no existing test cases for json parsing so I added a few.

Use specific url prefix for metrics test

Also changed the output to show client node IP addresses

Fixup mixed tabs in script for Vagrantfile demo

This commit replaces a hard tab with two spaces to match the intendation
of the rest of the defined script