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.
I think I'd prefer it if the "isRunningInLXC" call was next to the modprobe call.
Then it's more obvious why ensureVLANModule can't be called if we
are running inside LXC (there's no high level reason why we can't
do it AFAICS). Perhaps pass nw.Tag into ensureVLANModule
and have it return a bool to indicate whether it worked or not?