common, hostname: Allow dots in static hostname
https://www.freedesktop.org/software/systemd/man/latest/hostnamectl.html#hostname%20%5BNAME%5D
Static Hostnames can be fully qualified.
Otherwise we always replace dots with hyphens, even when the desired result is a dot separated FQDN.
Various system services expect a FQDN, such as Idm/IPA.
Related to #2918