Tom Pollard authored Nov 28, 2018 and Jürg Billeter committed Nov 30, 2018

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: Move _cached_buildtree() to be non local private method,
use _KeyStrength types to reduce duplication.

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

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

Closes #774

See merge request !978