rbd mirroring CLI calls are async and it doesn't wait
for the operation to be completed. ex:- `rbd mirror image enable`
it will enable the mirroring on the image but it doesn't
ensure that the image is mirroring enabled and healthy
primary. The same goes for the promote volume also.
This commits adds a check-in PromoteVolume to make sure
the image in a healthy state i.e `up+stopped`.
note:- not considering any intermediate states to make
sure the image is completely healthy before responding
success to the RPC call.
Signed-off-by: Madhu Rajanna <madhupr007@gmail.com>