Fixes NTP advices

Previously, the install-guide told about using the "ntpdate"
client in a cron job to update the time, which is a bad
advise. Also, it didn't tell how to install ntpdate either.
Using the ntp daemon is much better. This patch fixes that.

Change-Id: I6f624cf89d558bffcaee724e13274466a8fd3e2b
backport: havana
Merge "Fixes NTP advices"