OpenStack in general uses git tags to drive versioning of artifacts.
This is important because of our code review driven workflow, since
patches don't necessarily land in a predictable sequence, so flows
requiring a version number in a file to be updated at a specific
time are problematic.

By having pbr generate the correct version number then putting that
into the galaxy.yml file, we can match the behavior without too much
undue burden.

Also ignore build_artifact directory.

Change-Id: Id02fee682fb5a4b6fd6dcb0644848e6d1269e19b