ceph-disk: do not use mount --move (or --bind)

ceph-disk: do not use mount --move (or --bind)

The kernel does not let you mount --move when the parent mount is
shared (see, e.g., https://bugzilla.redhat.com/show_bug.cgi?id=917008
for another person this also confused).  We can't use --bind either
since that (on RHEL at least) screws up /etc/mtab so that the final
result looks like

 /var/lib/ceph/tmp/mnt.HNHoXU /var/lib/ceph/osd/ceph-0 none rw,bind 0 0

Instead, mount the original dev in the final location and then umount
from the old location.

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