_context.py: Add cache_buildtrees global user context, the default
of which is set to by default to 'always' via the addition of
cache-buildtrees to userconfig.yaml cache group. 'failure' & 'never'
can be given as valid options.

app.py & cli.py: Add --cache-buildtrees as a bst main option, which
when passed with a valid option can override the default or user
defined context for cache_buildtrees.

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

not _cached_sucess() could resolve to true if the element wasn't
cached at all. switch to _cached_failure() to ensure condition
reflects expected artifact state

Artifacts can be cached explicitly with an empty `build tree` when
built via the cli main options or user config for all or only
successful build artifacts. Default behaviour is to still create
and cache all expected buildtrees.

element.py: _cache_artifact() Check if context for cache_buildtrees
has been set to always or failure with a corresponding build
result, if not skip attempting to export the build-root. Element
types without a build-root are cached with an empty buildtree
regardless. Update _stage_sources_at() to warn the user that the
buildtree import is empty.

tests/integration: Add test to artifact.py for the optional caching
of buildtree content from bst build. Rename build-tree.py to
shellbuildtrees.py to reflect included test cases, add test for
empty buildtree warning and failure option.

NEWS: Add entry for new option.

Optional creation of buildtrees

Closes #896

See merge request !1135