Don't pull artifact build trees by default.

The addition of cached build trees being included in element
artifacts has led to mostly redundant download overheads when
pulling from a remote artifact server. As such the default behaviour
of pull shouldn't fetch the build tree object if available.

pullqueue.py: add extensible attributes to determine required/
excluded directories for element pull jobs. scheduler context
is used to determine pull_build_trees context.

element.py: extend relevant pull logic for specified subdir
consideration and ensure push logic does not lead to partial
artifact pushes. Change assumption that buildtree can be
extracted if artifact is cached.

_artifactcache/: artifactcache.py & cascache.py inclusion of
helper functions for subdir artifact checking & fetching, fetch
logic extended to only pull required artifact directories.
extract & checkout updated to handle full/partial operation.

tests/: addition of integration test pullbuildtrees.py,
buildtrees.py adapted cli options, testutils/artifactshare.py
has_artifact changed to return artifact digest on true condition.