Add ntp

Created by: dtu-compute

ntp is installed by

yum -y install ntp
timedatectl set-timezone Europe/Copenhagen
systemctl enable ntpd
systemctl restart ntpd

The production host will always be hosted at DO, so it should use either the default ntp servers or DO's own, if they have such.

devel[0-9] and staging should use ntp.compute.dtu.dk instead of CentOS's default

server 0.centos.pool.ntp.org iburst
server 1.centos.pool.ntp.org iburst
server 2.centos.pool.ntp.org iburst
server 3.centos.pool.ntp.org iburst