Optional GitLab Runner

Creates a GitLab runner on each hypervisor that has a registration token. Runner uses libvirt to spin up VMs to run the jobs in, with the base image /var/lib/libvirt/images/gitlab-runner-base.qcow2. I've been manually putting a debian-based runner image there. Guest must have the qemu guest agent, and accept SSH connections for user debian with the SSH key provided in the cloud config.