Allow user customize wg.conf file

[sckol]

Now etc/wireguard/wg.conf.j2 template is hardcoded into the role. This PR proposes allow user change the template to implement some custom network layout. Unortunately, it is impossible in Ansible without changing the role, see: https://stackoverflow.com/questions/55742881/is-there-a-way-to-override-a-template-defined-into-an-ansible-galaxy-role

[githubixx]

Thanks for the PR! But I already came to the conclusion that this doesn't makes that much sense. Also see https://github.com/githubixx/ansible-role-wireguard/pull/133 . The whole logic basically is in this template. If someone needs a different logic then I think it's better to fork this role.