Semantic version support for pbr

OpenStack uses semver but pbr doesn't actually understand it today.
Teaching it to understand semver will allow less thought and more
automation by developers, as well as better and clearer version
numbers for folk doing continual deployment of OpenStack.

Change-Id: Ifeb73fdd6a8acf3bf313f28a5079830c2b075b7a
Merge "Semantic version support for pbr"