Rename rbd.py to rbd_utils.py in libvirt driver directory

In libvirt driver directory, rbd.py confict with global rbd library
which is imported in rbd.py, so we rename rbd.py to rbd_utils.py.

Change-Id: Ib62e430e678fe09c4a8475a636a8ecc11a194f5c
Closes-Bug: #1352595