Fix rbd backend not working for none admin ceph user

The 'rbd_user' option allows nova administrators to override the default user
account used for RBD operations, with one that has potentially lower
privileges.  Not all parts of the Nova code honoured the 'rbd_user' option,
which resulted in failures when attempting to use a lesser privileged user for
RBD. This fix ensures the '--id' and '--config' parameters are passed to the
RBD command line tools in all cases.

Change-Id: Id99aa303791143360ad78074184583048e4878f0
Close-bug: 1255536
Merge "Fix rbd backend not working for none admin ceph user"