mon: mon_cluster_log_file_level

By popular demand, control the min level of logging that goes to
/var/lib/ceph/ceph.log.

Signed-off-by: Sage Weil <sage@inktank.com>
Merge pull request #502 from ceph/wip-mon-log-level

mon: mon_cluster_log_file_level

Reviewed-by: Samuel Just <sam.just@inktank.com>