ceph.conf: lower mon disk avail warning threshold

Only wanr when we hit 90% instead of default 70%

Signed-off-by: Sage Weil <sage@inktank.com>
(cherry picked from commit cf4bf09b2c4dae034332fb893cf96ed31adb7a4b)
ceph-deploy: stop daemons, archive, then purge[data]

Purge removes logs, and we want to archive those, so explicitly shut down
all daemons before doing the archiving step.

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