Add libselinux-python to beakerlib role requires
    
    If a beakerlib test is run on a fresh system with Standard Test Roles
    installed it failes with the following errror:
    
        Aborting, target uses selinux but python bindings
        (libselinux-python) aren't installed!
    
    This is caused by missing libselinux-python package. It would be nice
    if Standard Test Roles take care of installing this dependency.