libvirt: Support IPv6 with LXC

Libvirt's LXC implementation exposes a read-only  `/proc/sys/net` to the
guests. This causes some of the guest's default network configuration scripts
to fail.

This patch works-around the issue by using `post-up` hooks to configure IPv6.

Closes-Bug: 1340791

Change-Id: I805cad98d855fcb2c90b07e98ad3653d1620bd42
Fix Cells ImagePropertiesFilter can raise exceptions

In a misconfigured scenario the ImagePropertiesFilter can raise an
exception which causes the image to be unbuildable in the region. This
happens when the capabilities is set for prominent_hypervisor_version
to an empty list.

Change-Id: I17bd9d87fa69ad34b19591018543a34dfd050978
Closes-Bug: 1343462
Merge "Fix Cells ImagePropertiesFilter can raise exceptions"
Merge "libvirt: Support IPv6 with LXC"