BatchSpawnerBase: Replace {host} with self.ip in connect_to_job_cmd.

For implementations other than condor_ssh_to_job, it might be useful
to use the hostname on which the notebook was spawned to proxy it.

Author: olifre

batchspawner/batchspawner.py

@@ -175,7 +175,7 @@ def _req_keepvars_default(self):
     connect_to_job_cmd = Unicode('',
         help="Command to connect to running batch job and forward the port "
              "of the running notebook to the Hub. If empty, direct connectivity is assumed. "
-             "Uses self.job_id as {job_id} and the self.port as {port}."
+             "Uses self.job_id as {job_id}, self.port as {port} and self.ip as {host}."
              "If {rport} is used in this string, it is set to self.port, "
              "and a new random self.port is chosen locally and used as {port}."
              "This is useful e.g. for SSH port forwarding."
@@ -221,6 +221,7 @@ async def connect_to_job(self):
         a new random self.port is chosen locally (useful e.g. for SSH port forwarding).
         """
         subvars = self.get_req_subvars()
+        subvars['host'] = self.ip
         subvars['job_id'] = self.job_id
         if '{rport}' in self.connect_to_job_cmd:
             self.rport = self.port