mon: allow syslog level and facility for cluster log to be controlled

Allow user to control the minimum level to go to syslog for the client-
and server-side submission paths for the cluster log, along with the syslog
'facility'.  See syslog(3) man page.

Also move the level checks into a LogEntry method.

Closes: #3704
Signed-off-by: Sage Weil <sage@inktank.com>
Reviewed-by: Joao Luis <joao.luis@inktank.com>