Enable listening on specific interfaces

If you wish for ntp to listen on specific interfaces, instead of leaving it to the daemon's defaults, then:

1. either set the environment/node attribute ["ntp"]["listen"] to the specific value you want it to be
2. or set the environment/node attribute ["ntp"]["listen_network"] to one of the following:
    - "all"       sets the daemon to listen on "0.0.0.0"
    - "localhost" sets the daemon to listen on "127.0.0.1"
    - "primary"   sets the daemon to listen on node["ipaddress"]
    - a CIDR like "192.168.4.0/24" will set the daemon to listen on any server address found that matches that CIDR