Make versions aware of enabled pipelines.

Updates to make our versions controller a bit smarter so
that it only returns information on API versions which are
actually running.

With these changes a user can disable the v2.0 or v3 API
versions in their pipeline, restart keystone, and then have
versions return information only for the versions which
are actually running.

This is important because auth_token now uses info from the
keystone versions controller (in some cases) to dynamically
select an API version.

Fixes LP Bug #1158470.
Change-Id: I0fa8a82f08e7247c44fb7f4ff8dbb7d4ad58b9cc
Merge "Make versions aware of enabled pipelines."