rbd: destination pool should be source pool if it is not specified

rbd: destination pool should be source pool if it is not specified

Currently if user perform image rename operation and user give pool
name as a optional parameter (--pool=<pool_name>) then currently
its taking this optional pool name for source pool and making
destination pool name default pool name.
With this fix if user provide pool name as a optional pool name
parameter then it  will consider both soruce and destination pool
name as optional parameter pool name.

Fixes: http://tracker.ceph.com/issues/18326
Reported-by: МАРК КОРЕНБЕРГ <socketpair@gmail.com>
Signed-off-by: Gaurav Kumar Garg <garg.gaurav52@gmail.com>