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

i.e. with a given directory and filename, check parent directories until
either a directory with the filename is found, or you reach the root of
the filesystem.

This is a part of #222

WorkspaceProject contains all the information needed to refer back to a
project from its workspace (currently this is the project path and the
element used to create this workspace)

This is stored within a WorkspaceProjectCache object, which keeps
WorkspaceProjects around so they don't need to be loaded from disk
repeatedly.

Workspaces has been extended to contain the WorkspaceProjectCache, and
will use it when opening and closing workspaces.

Context has been extended to contain a WorkspaceProjectCache, as there
are times when we want to use it before a Workspaces can be initialised
(looking up a WorkspaceProject to find the directory that the project is
in)

This is a part of #222

This is a part of #222

This is a part of #222

Searching for the workspace file is preferred to searching for the
project file for the moment, because it avoids the circumstance of
the workspaces being inside a directory where a higher-level
ctory
may have project.conf (but may not be a buildstream project).

This is a part of #222

This is a part of #222

This is a part of #222

This is a part of #222

cli: Interactively warn if the user is trying to close the workspace they're using to load the project

This involves changes in:
* _stream.py:
  * Add the helper Stream.workspace_is_required()
* userconfig.yaml:
  * Add a default value for prompt.really-workspace-close-project-inaccessible
* _context.py:
  * Load the prompt 'really-workspace-close-project-inaccessible' from
    user config.
* cli.py:
  * If buildstream is invoked interactively, prompt the user to confirm
    that they want to close the workspace they're using to load this
    project.

This is a part of #222

This is a part of #222

This is a part of #222