Add OpenStack cloud provider

So far only implements Instances() interface.
There's some simple unittests above, but for some of these latter ones I needed some sort of (mocked) OpenStack service to thow requests against.  Since I wasn't looking forward to that and I wanted to verify my understanding of gophercloud anyway, I just added this and tested against a handy real OpenStack cluster.

I added a short comment explaining that, but I know I'm still relying on the fact that OpenStack users will already recognise these variables and know what to do.  If you want more explanation for k8s/non-openstackers stumbling across this, let me know.