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.
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.
