improve terraform handling of virtual machine domains

machine.domain has been an mandatory attribute which creates .archlinux.org entries automagically. For migrations this can be a problem if .archlinux.org supposed to point to a different machine. Adding the new machine with the same domain leads to additional DNS entries. The DNS ends up pointing to both machines in this case. By making the domain attribute optional for machines, we allow the new machines to be created without any domain pointing to them.