Add API version command

* 'api show' displays various bits about the API versions available in
  the client and on the servers.

Change-Id: I02452f62ee0cd698594bbcf90c0654234d86257a
Merge branch 'image-v1' into dt-test
Merge branch 'metadata-format' into dt-test
Merge branch 'api-list' into dt-test
Merge branch 'volume-keystone' into dt-test