gitlab_runner: Add VM based executor (libvirt-executor)

For some workloads running in a container is too restrictive, ex:
arch-boxes (loop device, filesystem mount, pacstrap) and archiso
(pacstrap). Currently they both run a TCG accelerated QEMU VM, which is
very slow and painful to work with. We should provide a better option to
our users!

This adds a hardware accelerated VM for this kinds of workloads, which
is way faster and you can do whatever you like (mostly)!

Fix #283

---

TODO:

• Enable KSM?
• Get archiso to build
  • Done: https://gitlab.archlinux.org/klausenbusk/archiso/-/pipelines/10976
• Use BUILD_FAILURE_EXIT_CODE and SYSTEM_FAILURE_EXIT_CODE: https://docs.gitlab.com/runner/executors/custom.html#error-handling
• Fix existing users to use the secure-vm tag

roles/gitlab_runner/files/libvirt-executor

@@ -56,16 +56,18 @@ wait_for_vm_shutdown() {
 
 # Create a updated VM image with the required tools
 create_vm_template() {
-    # Download from arch-boxes when
-    # https://gitlab.archlinux.org/archlinux/arch-boxes/-/merge_requests/172 is in
+    # FIXME: Download from arch-boxes when [1] is in
+    # [1] https://gitlab.archlinux.org/archlinux/arch-boxes/-/merge_requests/172
     local vm_name="libvirt_executor_vm_template_$(date +%s)_tmp"
-    cp Arch-Linux-x86_64-cloudimg-20210815.31636.qcow2 "${LIBVIRT_DEFAULT_POOL_PATH}/${vm_name}.qcow2"
-    qemu-img resize "${LIBVIRT_DEFAULT_POOL_PATH}/${vm_name}.qcow2" 10G
+    local image_path="${LIBVIRT_DEFAULT_POOL_PATH}/${vm_name}.qcow2"
+    trap "rm -f ${image_path}" EXIT
+    curl -sSf "${MIRROR}/images/v20210815.31636/Arch-Linux-x86_64-cloudimg-20210815.31636.qcow2" --output "${image_path}"
+    qemu-img resize "${image_path}" 10G
     # TODO remove exit 1 andre steder
     trap "virsh destroy ${vm_name}; virsh undefine ${vm_name} --remove-all-storage; exit 1" EXIT
     virt-install --name "${vm_name}" \
                  --cloud-init user-data=$PWD/user-data \
-                 --disk path="${LIBVIRT_DEFAULT_POOL_PATH}/${vm_name}.qcow2",device=disk \
+                 --disk path="${image_path}",device=disk \
                  --os-type Linux \
                  --os-variant archlinux \
                  --network network=default,filterref.filter=clean-traffic \