Fix error message about Service[ntp] in 'absent' case

If `Class[ntp]` is used with `ensure => absent` Puppet runs will fail with the following error:

    Error: /Stage[main]/Ntp/Service[ntp]: Could not evaluate: Could not find init script for 'ntp'

This happens because Puppet tries to evaluate the state of `Service[ntp]` although `Package[ntp]` (and with it the `ntpd` binary and the init scripts) hasn't been installed.

This patch solves the problem by only declaring `Service[ntp]` in the case `ensure != absent`.