Pass rbd_user id and conf path  as part of RBD URI for qemu-img

Unlike rados.py, qemu-img does not use id from CEPH_ARGS and has to be
given a ceph user id and conf path explicitly in RBD URI

Closes-Bug: #1261675
Change-Id: I9226515f2a860ed45d12012fd5b6bc7e842f6d32
Merge "Pass rbd_user id and conf path  as part of RBD URI for qemu-img"