resolves #9839
resolves #6929
resolves #6910

e2e: template env interpolation path testing

* systemd units require absolute paths
* ensure directory exists for dnsmasq

build: upgrade from Go 1.15.5 -> 1.15.6

The client allocation GC API returns a misleading error message when the
allocation exists but is not yet eligible for GC. Make this clear in the error
response.

Note in the docs that the allocation will still show on the server responses.

Also updated scripts/update_golang_version.sh to handle new CircleCI
file layout.

docs: add PreserveCounts param to job API doc.

Don't bootstrap when `bootstrap_expect` is zero

Fixed typo in index.mdx

* operator subpages

* line wrapping, fixed

docs: added lifecycle example to JSON Jobs API docs

docs: more documentation for lifecycle stanza

Co-authored-by: Tim Gross <tgross@hashicorp.com>

Co-authored-by: Tim Gross <tgross@hashicorp.com>

* add job register preservecounts param, update job versions response

* include job version diffs param

* update

docs: added examples of device responses to api docs

* Set 'only' ALL_TARGETS rather than append

This is functionally no different than before, but it's more correct.

* Re-scope VERBOSE=true

Previously this was only set when the OS was Linux; this was added in
805ade7d.

* Warn about unsupported OS rather than error

Also:

* Only print the warning when trying to build Nomad
* Print correct list of supported OSes

* Add alt text to Nomad logo
* Hoist Resources
* Use description from nomadproject.io
* Getting Started -> Quick Start
* Add production cluster section to Quick Start
* Remove "Who Uses Nomad"
* Move Contributing to contributing/README.md
* Add link to homepage from hero image

Although this behavior is unfortunate, we want to document the existing
behavior rather than break backwards compatibility.

UI: Quietly ignore orphaned allocs in the topo viz