Fixed lp:1353443 - no modprobe in lxc containers

Since the introduction of the networker, in some cases we
might encounter a Permission denied error when trying to
run "modprobe 8021q" (i.e. load the driver for VLAN support).
This happens only when the worker is running inside an LXC
container. With this fix, this doesn't happen anymore.
Not quite sure why nw.tag isn't a names.MachineTag actually.