Ansible: source the list of admins and users from gateway.yml

Right now, the list of admins' ssh keys can be found in gateway.yml while the list of wireguard keys can be found in /ansible/roles/wireguard/templates/wg0.conf.j2.

Let's try to unify that by creating a list of users in the playbook, so that it can be overridden easily:

users:
  mupuf:
    contact_info: "martin.roukala@mupuf.org, mupuf on IRC"
    ssh:
      public_key: ""
    wg:
      ip: 10.42.100.2/32
      public_key: ""
  user1:
    contact_info: "..."
    wg:
      ip: 10.42.100.3/32
      public_key: ""