Add Version Responses Consistent with Openstack

Barbican currently returns one-off version information for GETs to the
root resource. This blueprint calls for replacing that response with
an OpenStack consistent response based on json-home, that includes
which API versions are currently supported. Such version information
could facilitate automatic discovery of services when appended with
the endpoint information retrieved from Keystone service catalogs for
example.

Change-Id: I06a87a3d0a08b73cf5078988fce558dfa7c67416
Implements: blueprint fix-version-api
Merge "Add Version Responses Consistent with Openstack"