Be explicit about our NTP servers

*.uk.pool.ntp.org servers come from http://www.pool.ntp.org/en/use.html
and are A or AAAA records with a short TTL which point to different
servers. Also seem to use Anycast (although my networking knowledge is
sketchy).

ntp.ubuntu.com is a similarly short-lived A record.

time.euro.apple.com is what my Mac uses ;)
Merge pull request #585 from alphagov/feature/ntp-server-list

Be explicit about our NTP servers
Use explicit defaults for NTP checks

Using the defaults as in https://github.com/sensu/sensu-community-plugins/blob/c6cef387d980e17ba4e493f9cd48671c3b66ecf7/plugins/system/check-ntp.rb
but having the explicitly here so that people don’t have to go looking
elsewhere for the values.

I’ve asked in the #sensu channel on freenode for more documentation
about the the check-ntp plugin, but for now, this should quieten our
alerts and we can tune them in the future.