ceph-disk: fix mount options passed to move_mount

Commit 6cbe0f021f62b3ebd5f68fcc01a12fde6f08cff5 added a mount_options but
in certain cases it may be blank.  Fill in with the defaults, just as we
do in mount().

Backport: cuttlefish
Reviewed-by: Dan Mick <dan.mick@inktank.com>
Signed-off-by: Sage Weil <sage@inktank.com>
ceph-disk: do not mount -o ''

Let's not trust all distros to be polite about this.

Signed-off-by: Sage Weil <sage@inktank.com>