Lookup tenant id via keystone_tenant.instances

 * Previously tenant_id was being looked up via the catalog,
   but that precludes the tenant being created in the same run
   as the quantum resource.  Using instances ensures that the id
   can always be looked up by the name.

Change-Id: I038edc10e38c4edacb4f40c446bf75156b2af882
Merge "Lookup tenant id via keystone_tenant.instances"