[COOK-4339] Disable MONLIST by default

After the huge CloudFlare DDoS using ntp, they've published a followup:
http://blog.cloudflare.com/technical-details-behind-a-400gbps-ntp-amplification-ddos-attack

One of the things mentioned is to ensure MONLIST is disabled in your ntp
servers. While ntp may or may not be exposed for any given converge, it
may be appropriate to get this setting into the attribute/config and
have it disabled by default just in case: http://openntpproject.org/

https://tickets.opscode.com/browse/COOK-4339