Add support for tag based versioning.
Add support for tag based versioning.

Versions driven by git.
If the current revision is tagged, then that's the version cut.
If not, for post-versioning, the current version is the tag plus an count of the
revs since the tag.
For pre-versioning, it's the logic that's in create-tarball.sh.
Additionally, a versioninfo file is added to the tree so that pkg-resources
can find out what the version is.

Change-Id: I5991345735fa14666305538c556b519160b02b4b

markmc/oslo.packaging,markmc/oslo.packaging