Support live-migration out of the box

This configures nova to use live-migration as default.

If the hypervisor don't contact each other on the mgmt-interface this
needs additional firewall configuration:

iptables -I INPUT 10 -p tcp -s <ip of other hypervisor> --dport 16509 -j ACCEPT
iptables -I INPUT 10 -p tcp -s <ip of other hypervisor> --dport 49152:49215 -j ACCEPT