Create a role for Ansible user

[nymous]

That way we can use it in the dedicated create-ansible-user playbook for a short first setup, and reuse it in the "setup everything" playbook that we run regularly to ensure the user is always configured as we want it.

For example, we could harden the SSH authorized key by adding the from= option, to only allow authentication from the Ansible host (see man sshd).