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