use_artifact_config added as an optional default arg, allowing
for loading of given elements artifact remote config.

Provide bst shell --use-buildtree the ability to attempt to
acquire missing buildtrees, given respective option, user
pull-buildtree context and remote availability.

_frontend/cli.py: Refactor logic for determining --use-buildtree
option with given opportunity to attempt pulling a non-local
buildtree. Element loaded with artifact_config to allow remote
querying.

_stream.py: With given user option and element state, construct
PullQueue to fetch remote buildtree. Continue or Error without
buildtree if cannot be attained.

tests/integration/build-tree.py: Update to support new usecases

Download buildtrees on demand for bst shell --use-buildtree

Closes #829

See merge request !1050

This is a breaking change, as it affects behaviour that people might be
relying on. An entry has been added to NEWS.

As proposed on the mailing list, this change removes the unconditional
prompts on:

    o: bst workspace reset
    o: bst workspace close --remove-dir

If interactive, these commands would always interrupt you with a prompt
like this:

    This will remove all your changes, are you sure?

This seems like it may just save someone's work some time. It may also
condition folks to hit 'y' quickly without thinking.

This change also makes the non-interactive behaviour consistent with the
interactive behaviour in the default case. There is also the case of the
prompt configured by 'really-workspace-close-project-inaccessible',
which may be tackled in later work.

This change also removes the new config options to suppress those
prompts, and their associated news entry.

The relevant bit of the mailing list conversation is here:
https://mail.gnome.org/archives/buildstream-list/2018-December/msg00106.html

The issue to make interactive and non-interactive behaviour consistent
is here:
#744