Allow setting Keystone API version in service catalog

This is the first step toward removing the Identity API version from the
Service Catalog, by setting IDENTITY_API_VERSION="" in local.conf
no version is added to the Identity Service Catalog URLs.

This _will_ break the current clients but is useful for testing
API version discovery.

Change-Id: I4870cddcb75488b3b6837a15b063aa69cdcc1444
Merge branch 'identity-api-version' into dt-test