buildserver instances for contributors

@licaon-kter and @Rudloff (and potentially @relan?) had interest in having access to a buildserver instance on the gcc buildfarm servers.

We have enough resources there, it's just a question of getting people access there in some form or another.

Through @uniqx's nested libvirt ansible setup this should be fairly straighforward, but let's recap the steps.

I'll create a VM instance for each contributor wanting access, this can happily run makebuildserver overnight then. Those who want access should send in an ssh public key. I think posting here might be sufficient, as they'll only get access to a VM instance.

I'll then add these ssh keys to the VM instances' root account.

Now this is where it gets a bit hazy. There'll need to be some port forwarding into the VMs. Is that setup automatically, where would I see the ports per VM? Or do I need to configure this manually?

Or do I somehow mis-remember the whole setup and this is not going to work like this at all?

Ping @eighthave for review.