Clear NSS session cache when socket is closed

Even when NSS connection is closed, there may be still cached
certificates in the NSS lib. This may cause subsequent NSS
initialization to crash. This problem especially reproduces in the
unit tests.

Reviewed-By: Petr Viktorin <pviktori@redhat.com>

ipapython/nsslib.py

@@ -247,6 +247,7 @@ class NSSConnection(httplib.HTTPConnection, NSSAddressFamilyFallback):
         if self.sock:
             self.sock.close()   # close it manually... there may be other refs
             self.sock = None
+            ssl.clear_session_cache()
 
     def endheaders(self, message=None):
         """