In element, platform asks for host os and architecture to default to when
SandboxConfig is initialised.
This changes element cache keys so those have been updated in the tests.

Also update tests to be consistent with this

These are contained within the platform field.

Same format as OptionArch tests, but with OS's.

  o _platform/linux.py: Add linux32 flag to send to sandbox bwrap when the
    build arch is x86-32 and the machines arch is x86-64 or similarly with
    aarch32 and aarch64.
  o sandbox/_sandboxbwrap.py: Use flag passed to start bwrap command with
    linux32 if set.

Execution environment reqs

Closes #775

See merge request !969

Get cascache.py to use this

Stops unneccesary pushing of builds that have already been built, just checks
the action cache to begin with.

Fixes #628

The endpoint may not necessarily be the same as execution-service endpoint.

Part of #628

Part of #628

Remote-execution client flow optimisation

Closes #628

See merge request !982

Instead of duplicating the whole test body, run it twice while
checking for a warning or an error depending on the parameter.

This also ensures it is exposed via the main buildstream __init__.py
file, fixes the imports of CoreWarnings, and adjusts the documentation
links to point to the right place.

Now the commands have documentation linkability so we can link to
them when referring to them anywhere in the docs. This reference
was previously referring to a bogus non-existent `build-stream track`,
maybe this dates way back to the days before we named the frontend `bst`.

Fixes the configurable warning part of this to properly link
to the project.conf documentation describing what configurable
warnings actually are.

Also fix the plugin raise the actual configurable warning which
it advertizes that it raises; which is 'inconsistent-submodule',
not 'inconsistent-submodules'.

Change 'INCONSISTENT_SUBMODULE' for 'WARN_INCONSISTENT_SUBMODULE' to
make things a bit more readable and comprehensive.

Misc cleanups

See merge request !993