Consider enabling hostname service on subsequent boots

[bgilbert]

afterburn-hostname.service currently only runs on the first boot, but some cloud platforms may change instance hostnames on subsequent boots (e.g. after the instance is stopped and started). Consider enabling the service on subsequent boots, perhaps only on platforms that need it.

Where an instance metadata service is used, this would require the OS to activate networking in the initrd on subsequent boots.

xref coreos/fedora-coreos-config#2176