refuse to purgedata if ceph is installed

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
go back to removing /etc/ceph completely

Signed-off-by: Alfredo Deza <alfredo.deza@inktank.com>
Merge pull request #141 from alfredodeza/6853

remove /etc/ceph

Reviewed-by: Sage Weil <sage@inktank.com>