Add the heartbeat period to the registration reply

The heartbeat period can then be used to set the check period on the
engine side.

IPython/parallel/controller/hub.py

@@ -904,7 +904,7 b' class Hub(SessionFactory):'
 
         self.log.debug("registration::register_engine(%i, %r)", eid, uuid)
 
-        content = dict(id=eid,status='ok')
+        content = dict(id=eid,status='ok',hb_period=self.heartmonitor.period)
         # check if requesting available IDs:
         if cast_bytes(uuid) in self.by_ident:
             try: