lxc: don't up the veth interfaces unless explicitly asked to

lxc: don't up the veth interfaces unless explicitly asked to

Upping an interface for no reason and not configuring it is a cardinal sin.

With the default addrgenmode if eui64 it sticks a link-local address to the
interface. That is not good, as NetworkManager would see an address configured,
assume the interface is already configured and won't touch it iself and the
interface might stay unconfigured until the end of the days.

Resolves: https://bugzilla.redhat.com/show_bug.cgi?id=1124721

Signed-off-by: Michal Privoznik <mprivozn@redhat.com>