Make Ubuntu 13.04 (raring) the default release

Ubuntu 13.04 has been released now for 3 months. The updated libvirt,
openvswitch and kernel are all beneficial to various OpenStack components,
and many other software is updated beyond the versions in Ubuntu 12.10.

Change-Id: I358aed8bf906c3ff5103f19b1f9e6ac689b5d5ee