Introduce a way on the connection to check the client version

Author: cderici

juju/client/connection.py

@@ -360,6 +360,12 @@ def username(self):
             return None
         return self.usertag[len('user-'):]
 
+    @property
+    def is_using_old_client(self):
+        if self.info is None:
+            raise errors.JujuError("Not connected yet.")
+        return self.info['server-version'].startswith('2.')
+
     @property
     def is_open(self):
         return self.monitor.status == Monitor.CONNECTED