Remove hard coded ntp servers in configuration file

Because its not always appropriate to be using pool servers (security
concerns, network reliability, aggressive perimeter firewalling) I have
removed the default Ubuntu related server entries.

To ensure the role keeps working OOTB I've made them the default NTP
servers for the role.

This does mean instead of ntp_servers+5 defaults you only have ntp_servers, I'm not sure if this will be a deal breaker or not.