Handle config drives being stored on rbd

rbd is the only example of a currently supported image storage
backend where it makes sense to put the config drive in the
configured storage backend instead of local hypervisor disk.
Support this special case by moving the image to rbd once it has
been created in the local instance directory on the hypervisor.

Change-Id: Ia3ca5a18c79d62b71b9c042a612d12dd074b245e
Closes-bug: #1369627
Closes-bug: #1361840