Fix live migration for File/Volume based Instances

- Open libvirt ports for migration http://wiki.libvirt.org/page/FAQ
- Add python-novaclient for compute nodes to provide nova CLI
- Change libvirt_disk_cachemodes to"file=directsync","block=none".

This allows live block migration, as DirectSync mode is like writethrough, but it bypasses the host page cache.

Closes-Bug: 1329953
Change-Id: Ie7cee823fac70f9c2dd46c04f8651cec6bddccc4