Use constant for user and group patterns
    
    User and groups regexp are the same and constant should be used to avoid
    any future misconfigurations.
    
    https://fedorahosted.org/freeipa/ticket/5822
    
    Reviewed-By: Stanislav Laznicka <slaznick@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>