update source role documentation
    
    * source role already copies files to the VM, no need to define it on test playbook
    
    * there is no need to set tenv_workdir in the playbooks anymore
    
    * Add quotes to example to make it more clear.
      Ansible requires quotes when parameter is just the ansible variable, but it is optional if it is part of string.
    
    fix: https://pagure.io/standard-test-roles/issue/384