WIP: Don't pull artifact buildtrees by default

- Set default pull to not include buildtree artifact dir
- PullQueue configurable subdir attribute
- Add 'pullbuildtrees' global option to for user conf
- Add --pull-buildtrees flag to bst build cli
- Add --pull-buildtrees flag to bst pull cli
- Add helper function _fetch_subdir to cascache, to fetch buildtree
  or any other subdir digest
- Make element._pull_pending not assume no need to process pull if
  artifact is cached if buildtrees are set to be pulled
- Ensure cascache.py doesn't try to checkout/extract a dangling ref

ToDo:
- Tests