hostname: Playbook to set system hostname

The `hostname.yml` playbook does what it says on the tin: it sets the
hostname of the managed node. The `preferred_hostname` variable can be
used to specify the value to use for the hostname. If it is not set, the
name specified in the inventory is used.

hostname.yml

@@ -0,0 +1,5 @@
+- hosts: all
+  tasks:
+  - name: ensure hostname is set
+    hostname:
+      name={{ preferred_hostname|d(inventory_hostname) }}