Add simple playbook task for upgrading servers

We want to treat our servers as cattle; hopefully when this is fleshed out a bit more, it can accomplish the job without too many casualties.

playbooks/tasks/upgrade-servers.yml

@@ -2,6 +2,7 @@
 
 - name: upgrade and reboot all servers
   hosts: all,!rsync_net,!hetzner_storageboxes
+  serial: 20%
   gather_facts: false
 
   tasks: