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

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