add users without sudo (gh_user_no_sudo)

It's handy to be able to create users that *don't* have sudo rights. Here at Stanford we use this to add users to our VPC bastion box (ssh proxy) so these users can use this machine for ssh tunneling, but I
don't want to give those users the keys to the kingdom.

This let's me configure a playbook like this.

    roles :
      - common
      - supervisor
      - role: gh_users
        gh_users:
          - su1
          - su2
          - su3
        gh_users_no_sudo:
          - normal1
          - normal2

The new gh_users_no_sudo list can be empty.

We've used this at Stanford already and it seems to be working.