feat(user-data): enable time synchronization on startup

Enable the use of ntp in the system clock (is disabled by default) and set the timezone to UTC (CoreOS recommendation).

https://coreos.com/docs/cluster-management/setup/configuring-date-and-timezone/
http://www.freedesktop.org/software/systemd/man/timedatectl.html

closes #2518 

This is the output of `timedatectl` after the synchronization:
```console
      Local time: Thu 2014-12-04 01:02:18 UTC
  Universal time: Thu 2014-12-04 01:02:18 UTC
        RTC time: Thu 2014-12-04 01:02:17
       Time zone: UTC (UTC, +0000)
     NTP enabled: yes
NTP synchronized: yes
 RTC in local TZ: no
      DST active: n/a
```