Support for extending RBD volumes was added during Stein: https://review.openstack.org/#/c/613039/
Closing as a duplicate of 1184786 that will serve as our TestOnly tracker for this in 16 where we will be testing our iSCSI (block), NFS (file) and RBD (net) paths for handling this in Nova. *** This bug has been marked as a duplicate of bug 1184786 ***