Disable HTTP timeouts for dcos task exec` requests (#1099)

As this command relies on a long-running HTTP connection using chucked
transfer encoding, it is common to have no data to read from the server
for an undefined period of time.

This happens for example when opening a shell session and not typing
anything.

https://jira.mesosphere.com/browse/DCOS_OSS-1827

dcos/mesos.py

@@ -1302,6 +1302,7 @@ def _launch_nested_container_session(self):
         response = http.post(
             self.agent_url,
             data=json.dumps(message),
+            timeout=None,
             **req_extra_args)
 
         self._process_output_stream(response)
@@ -1420,6 +1421,7 @@ def _input_streamer():
         http.post(
             self.agent_url,
             data=_input_streamer(),
+            timeout=None,
             **req_extra_args)
 
     def _input_thread(self):