Add option to control ansible_managed in config

Most roles simply use # {{ ansible_managed }} as the top line of their
configuration file so in many cases thats what people will be looking for (ie,
me). For consistency with other roles/templates this option allows swapping
between the default cow saying 'Managed by Ansible' and '# {{ ansible_managed
}}'
Merge pull request #2 from goetzk/Make-cow-in-config-file-optional

Add option to control ansible_managed in config