more debugging and health checks for services

- log that external services are added (helps catch accidental external services due to missing fields)
- check connectivity of services with web endpoints periodically

jupyterhub/services/service.py

@@ -160,6 +160,11 @@ class Service(LoggingConfigurable):
     def managed(self):
         """Am I managed by the Hub?"""
         return bool(self.command)
+    
+    @property
+    def kind(self):
+        """Am I managed (as text)"""
+        return 'managed' if self.managed else 'external'
 
     command = Command(minlen=0,
         help="Command to spawn this service, if managed."