Add user namespace support for libvirt-lxc

This change adds two config values that allow deployers
to enable user namespacing for libvirt-lxc containers.
User namespacing allows the mapping of uids and gids in
a container to separate uids and gids outside of the
container. The main advantage of this is to allow a
container's processes to appear to be running as root,
while actually running as a different user on the host.

Implements: bp libvirt-lxc-user-namespaces

Change-Id: I5f517f228b477f8238eccbb604b9ffd0ebaa5927