ceph: fix purge

Signed-off-by: Sage Weil <sage@inktank.com>
ceph: install ceph-mds, ceph-common

Signed-off-by: Sage Weil <sage@inktank.com>
nuke: remove librados, librbd

Signed-off-by: Sage Weil <sage@inktank.com>
ceph: simpilfy package removal

apt-get doesn't have a nice way to tell if the package is not install and
we don't need to purge it.  Well, not one I found in 5 minutes.  Just
do a big purge and assume it works, or failed because there was nothing to
be done.

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