common: Add config shared by all hosts

The `common.yaml` Butane configuration file merges in all the other
various Butane configuration files that we want to share amonst all
CoreOS machines.  These include the authorized SSH keys list, collectd
deployment, SSH host certificate configuration, etc.

common.yaml

@@ -0,0 +1,11 @@
+variant: fcos
+version: 1.4.0
+
+ignition:
+  config:
+    merge:
+    - local: sshkeys.ign
+    - local: collectd.ign
+    - local: local_exporter.ign
+    - local: notify-shutdown.ign
+    - local: step-ssh.ign

k8s-aarch64-n0.yaml

@@ -3,8 +3,9 @@ version: 1.4.0
 ignition:
   config:
     merge:
-    - local: sshkeys.ign
+    - local: common.ign
     - local: kubelet.ign
+
 storage:
   files:
   - path: /etc/hostname

nvr1.yaml

@@ -4,13 +4,11 @@ version: 1.4.0
 ignition:
   config:
     merge:
-    - local: sshkeys.ign
+    - local: common.ign
     - local: zram.ign
     - local: gasket-driver.ign
     - local: frigate.ign
-    - local: collectd.ign
     - local: nginx.ign
-    - local: notify-shutdown.ign
 
 kernel_arguments:
   should_exist: