userspace/monit: use start/stop functionality of snmp init script

Signed-off-by: Adam Wujek <adam.wujek@cern.ch>

userspace/rootfs_override/etc/monit.d/snmp.conf

  check process snmpd with pidfile /var/run/snmpd.pid
    start program = "/etc/init.d/S80snmp start"
-   stop program = "/usr/bin/killall snmpd"
+   stop program = "/etc/init.d/S80snmp stop"
 #   if failed host localhost port 161 type udp then restart
 #   if 5 restarts within 10 then exec "/sbin/reboot"