There is nothing in ArtifactCacheSpec that's actually specific to
artifacts, so I've made it a CAS class so we can use the same
spec for remote execution.

Since the artifact cache and remote execution share the same
local CAS store, they should share the same CASCache object. Moving
this into context allows us to do this.

Uses the new config options introduced earlier to specify remote
exec storage and execution service.

Tests that we get a sensible error message when we specify incomplete
or wrong information in the configuration for remote execution.

The port number must come after the hostname and before the path.
The path is ignored by CASCache anyway, so including it is misleading.

Split remote execution from artifact cache

Closes #750

See merge request !946

setup.py: require ruamel.yaml >= 0.15.41 < 0.15.52

See merge request !975

Adds helper function _buildtree_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.

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