timedated: introduce systemd-timedated-ntp.target which is controlled by timedated...

We shouldn't hardcode the name of the NTP implementation in the
timedated mechanism, especially since Fedora currently switched from NTP
to chrony.

This patch introduces a new target that is enabled/disabled instead of
the actual NTP implementation. The various NTP implementations should
then add .wants/ symlinks to their services and BindTo back to the
target, so that their implementations are started/stopped jointly with
the target.

https://bugzilla.redhat.com/show_bug.cgi?id=815748