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>

ipapython/kerberos.py

@@ -181,3 +181,7 @@ class Principal(object):
             principal_string = u'@'.join([principal_string, realm])
 
         return principal_string
+
+    def __repr__(self):
+        return "{0.__module__}.{0.__name__}('{1}')".format(
+            self.__class__, self)