Specify '-e lxc' to use the LXC driver

I built boot2docker.iso with the latest commit and Docker v0.9.0.
Docker v0.9.0 in boot2docker-vm doesn't seem to work well.

Your team may be working for the new native driver of Docker v0.9.0,
but I patched a bit to use the LXC driver for now.
It seems to work fine so far.

I know it's not good for release, but just in case I'd like to put it here.