Use ubuntu Mitaka repo

The rabbitmq-server package is upgraded to 3.5.7 in cloud-archive
so we update centos to match

The xen-utils package now needs an explict version, xen-utils-4.6 is
what is provided by cloud-archive mitaka

Libvirt 1.3 is in the ubuntu cloud-archive. This has a new daemon for
logging that needs further implementation in newton. For now, it has
been disabled within the qemu.conf

Co-Authored-By: Jeffrey Zhang <jeffrey.zhang@99cloud.net>
TrivialFix
Change-Id: I84217824817c484b6800a64cbd7767b127a3098e