Recent change to devstack dropped installing test-requirements [1]
However, this caused gate failures due to lack of glance-store
deps for cinder and swift support.
This patch makes devstack install relevant extras depending on
enabled features.
Additionally, relevant functions are added/fixed to make this
possible.
glance-store = glance_store (for gerrit search match)
[1] https://review.opendev.org/715469
Change-Id: I0bf5792a6058b52936115b515ea8360f6264a7c9
be26306b4a