dracut: speedup setup + fix auto6/dhcp6 tests

Speedup setup by generating initramfs at the same time (to use all cores).

Fix auto6/dhcp6 tests by making /etc/machine-id the same. Otherwise DHCPv6 would send another address (or none if all addresses are used). Copying /etc/machine-id is fine, because by default, dracut generates initramfs in HostOnly mode, which makes sure /etc/machine-id is the same in initramfs and in the target root.