Improve tenant_hash usage in keystone_tenant.

 * The tenant_hash used by the keystone_tenant type to keep track
   of tenants is now updated with new tenants as they are created.
 * Added 'instance' helper method that returns
   tenant_hash[resource[:name]]
 * Added spec tests for the changes.

Change-Id: I077e5012a3f2d20d3df4cc55f1150490d4b31f58