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.
The same goes for the somewhat flawed flag "isVLANSupportInstalled". I'll just go ahead and rename ensureVLANModule to maybeLoadVLANModule and do both checks inside.