Fix a couple items in Barbican's container creation

* Ensure None is not added to the created_containers list,
which occasionally causes test tear-downs to fail as it tries
to delete all of the created_containers.
* Wrap the container creation response in a CloudkeepResponse.

Change-Id: I35531a027c2b84044e79ccf4c630d9a205b35289
Merge "Fix a couple items in Barbican's container creation"