Add ability to extract a list of versions

Add ability to extract a list of know versions from the service
catalog.  Currently at least, the service catalog contains versions.
The get_versions method returns either a list of versions or an
[] empty list if the service catalog doesn't contain a detectable
version.  If the service catalog does not contain a service, None
is returned.

Change-Id: I207553ca4bdd0083aff1802182039dde6ab72148
Merge "Add ability to extract a list of versions"