hostname: Also write /etc/hosts

This commit introduces a new role, *hostname*, that is used by the
`hostname.yml` playbook to set the hostname. It also writes
`/etc/hosts` using a template.

hostname.yml

@@ -1,5 +1,3 @@
 - hosts: all
-  tasks:
-  - name: ensure hostname is set
-    hostname:
-      name={{ preferred_hostname|d(inventory_hostname) }}
+  roles:
+  - hostname