Add UCA repo when building Ubuntu 12.04 images

Ubuntu 12.04 standard repos only contains libvirt-bin.0.9.8, but setup-network script fails if so :
 - --persistent option in virsh net-list is not present in 0.9.8
 - virtualport type='openvswitch' is not available in 0.9.8

Change-Id: I018447c1562edbf71c593785b9a17335c3687322