Add a new network for e2e tests, to isolate it from any other cluster.

@jbeda @filbranden please take a look.
I'm not sure that you can launch a VM on a network that isn't created yet so this may introduce a race.  I'd (a) move this up above where we create the `master-start.sh` script and (b) put a `wait` in for the network itself.  To do that the "wait and report errors" snippet below should probably be pulled out into a function.

I have a e2e tweak coming and I'm happy to make these changes there.