remote port selection: separate wrapper into separate traitlet

- Needed in cases where path to the wrapper may need explicit
  specification.

batchspawner/batchspawner.py

@@ -156,6 +156,12 @@ def _req_keepvars_default(self):
              "Must include {cmd} which will be replaced with the jupyterhub-singleuser command line."
         ).tag(config=True)
 
+    batchspawner_wrapper = Unicode('batchspawner-singleuser',
+        help="A wrapper which is capable of special batchspawner setup: currently sets the port on "
+             "the remote host.  Not needed to be set under normal circumstances, unless path needs "
+             "specification."
+        ).tag(config=True)
+
     # Raw output of job submission command unless overridden
     job_id = Unicode()
 
@@ -188,7 +194,7 @@ def parse_job_id(self, output):
 
     def cmd_formatted_for_batch(self):
         """The command which is substituted inside of the batch script"""
-        return ' '.join(['batchspawner-singleuser'] + self.cmd + self.get_args())
+        return ' '.join(self.batchspawner_wrapper + self.cmd + self.get_args())
 
     @gen.coroutine
     def run_command(self, cmd, input=None, env=None):