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.
Merge pull request #742 from edx/sef/gh_users_no_sudo_master

add users without sudo (gh_user_no_sudo)