It's inconvenient to have to create a Linux platform to parse the bwrap version
and we want to get the version in a consistent manner.

If `bwrap` fails to set up the sandbox and start the payload command
it won't write an exit-code in --json-status-fd,
so we can report if it was a sandboxing failure if we don't get exit-code status
and a payload command failure if we do and it's non-zero.

Closes #286

Distinguish between bubblewrap sandboxing failure and command failure

Closes #286

See merge request !868

of which is set to 'all'

_frontend/app.py & cli.py: Add --user-remotes as a bst main option,
with values of 'all', 'user' or 'none' which when passed will
override the default or user defined context for user_remotes

tests/completions/completions.py: Update for the added flag

servers.

By adding a new user context and bst main option, we can provide
more granular control of what remotes are utilised during relevant
bst pull and push actions.

_artifactcache/artifactcache.py: Expand setup_remotes() to consider
the user context value of 'use_remotes'. Allowing the user to emit
interacting with all, or all non user defined remotes.

tests/frontend: pull.py & push.py addition of tests for checking
expected behaviour. Ensures interaction with user, project or no
remotes.