Enhance __repr__ method of Principal
    
    `__repr__` now returns more descriptive string containing the actual principal
    name while keeping the ability to reconstruct the object from it.
    
    This makes principal names visible in debug logs, easing troubleshooting a
    bit.
    
    https://fedorahosted.org/freeipa/ticket/6505
    
    Reviewed-By: Christian Heimes <cheimes@redhat.com>