_stream.py: Ability to pull missing buildtrees outside of pull/build

Adds helper function _buildtree_pull_required() to determine if a
pullqueue should be constructed, for commands outside of bst pull
and build where it is determined that an element's buildtree
artifact is to be required given the respective semantics and
config. Utilised in push() to attempt to mitigate skipping the push
of partial elements without the user having to have preceded it with
an explicit pull.

cli.py: Add new behaviour to push command description

element.py: Add _cached_buildtree() to access private method, use
_KeyStrength types to reduce duplication.

tests/integration/pullbuildtrees.py also updated to cover this
use-case.