kubevirt: Set transient hostname from metadata

At KubeVirt the hostname is usually set using DHCP on the case of
fcos/rhcos but at some use cases this is not the case and at least the
transient hostname should be set from the metadata. This change runs the
afterburn service that read metadata and dump and env file that can be
used later on with hostnamectl to set the transient hostname.

Signed-off-by: Enrique Llorente <ellorent@redhat.com>

templates/common/kubevirt/units/afterburn-transient-hostname.service.yaml

@@ -0,0 +1,26 @@
+name: afterburn-transient-hostname.service
+enabled: true
+contents: |
+  [Unit]
+  Description=Set Transient Hostname from KubeVirt meta_data
+
+  # Run afterburn service for collect info from metadata server
+  # see: https://coreos.github.io/afterburn/usage/attributes/
+  Wants=afterburn.service
+  After=afterburn.service
+
+  # Before hostname is received from DHCP
+  Before=network-pre.target
+  Wants=network-pre.target
+
+  # Run before kubelet
+  Before=kubelet.service
+
+  [Service]
+  # Mark afterburn environment file optional, due to it is possible that afterburn service was not executed
+  EnvironmentFile=/run/metadata/afterburn
+  ExecStart=hostnamectl hostname --transient "${AFTERBURN_KUBEVIRT_HOSTNAME}"
+  Type=oneshot
+
+  [Install]
+  WantedBy=network-online.target