Support other providers

I just ran into a problem today that was provider-specific. In my case I use a local registry to pull an image created from my local build, and then work with the `libvirt-coreos` provider and the `kube-(up|down).sh` scripts.

In looking at [lease.sh](//github.com/deis/e2e-runner/blob/master/scripts/lease.sh), it seems that script could be refactored to use other providers.

Never worked with `bats` so have no idea about that.

Of course it's probably harder than it looks, like most things.