ceph-disk: use mount --bind instead of --move

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).  Instead, --bind to the new
location and lazily unmount the old one.  This fixes ceph-disk activate
on Fedora and (presumably) RHEL/CentOS.

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