kickstart: Allow to specifiy additional packages to install

This allows to customize packages for installation during kickstart
kickstart: Get rid of --ignore-missing

By moving ntpdate out of the hard coded list we can use the same
template for different centos/rhel versions. This can now be specified
via the generic data hash.

The --ignore-missing in the kickstart templates is a constant source of
trouble since it lets the installation fail silently.
Merge pull request #39 from agx/features/kill---ignore-missing

Features/kill   ignore missing