Make the identification of the primary master easier with 'neoctl print node'

Now, the primary master is the running master with None displayed in the
last column. Before, it could be the id timestamp of when it was secondary,
which was obsolete information.

neo/master/app.py

@@ -244,6 +244,7 @@ class Application(BaseApplication):
             self.uuid = self.getNewUUID(None, self.server, NodeTypes.MASTER)
             logging.info('My UUID: ' + uuid_str(self.uuid))
         self._node.setRunning()
+        self._node.id_timestamp = None
         self.primary = monotonic_time()
 
         # Do not restart automatically if an election happens, in order