rbd: accept --user, refuse -i

rbd: accept --user, refuse -i

--user is the equivalent of --id (mentioned in docs and ceph -h) and
used by at least a couple of qa scripts and teuthology.  Add it to the
list of valid rbd options - it's handled inside global_init() before
Shell:execute() is called.

OTOH allowing -i is bogus - global_init() wouldn't process it for
clients (CEPH_ENTITY_TYPE_CLIENT), which rbd tool is one of.  Drop it.

Signed-off-by: Ilya Dryomov <idryomov@redhat.com>