Fix exception on logout

When logging out, the session is obviously not logged
after the request to logout has been processed.  If
retry is False, do not be surprised we don't have a 'lastresponse'.

Change-Id: Ia5cbdab3736148fa36ced8835bd8d9fdca1dc12d

pyghmi/ipmi/private/session.py

@@ -630,7 +630,7 @@ class Session(object):
             Session.wait_for_rsp(timeout=timeout)
         lastresponse = self.lastresponse
         self.incommand = False
-        if lastresponse is None:
+        if retry and lastresponse is None:
             raise exc.IpmiException('Session no longer connected')
         return lastresponse