ceph-deploy: purge before archiving

Purge will uninstall and (in so doing) stop the daemons. This avoids trying
to tar up the mon data or logs while they are being written to, which
avoids errors like

2013-04-16T20:21:47.103 INFO:teuthology.task.ceph-deploy:Archiving mon data...
2013-04-16T20:21:47.545 INFO:teuthology.orchestra.run.err:tar: ./ceph-mira089/store.db/000009.log: file changed as we read it

Also drop the unnecessary uninstall (it is implied by purge).

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