Rename an internal method to _helper_list()

helper_list() is just a helper method for "list_<resoruces>" of
telemetry_client and it is used in telemetry_client only.
telemetry_client will be migrated to tempest-lib and we need to
clarify whether each method is external or internal.
This patch renames the method for clarifying an internal method.

Partially implements blueprint consistent-service-method-names

Change-Id: I12922cf1ca968eb1d9d793879aa22921cc39a56b
diff --git a/tempest/services/telemetry/json/telemetry_client.py b/tempest/services/telemetry/json/telemetry_client.py
index 36c123b..070309d 100644
--- a/tempest/services/telemetry/json/telemetry_client.py
+++ b/tempest/services/telemetry/json/telemetry_client.py
@@ -50,7 +50,7 @@
         body = self.deserialize(body)
         return service_client.ResponseBody(resp, body)
 
-    def helper_list(self, uri, query=None, period=None):
+    def _helper_list(self, uri, query=None, period=None):
         uri_dict = {}
         if query:
             uri_dict = {'q.field': query[0],
@@ -67,23 +67,23 @@
 
     def list_resources(self, query=None):
         uri = '%s/resources' % self.uri_prefix
-        return self.helper_list(uri, query)
+        return self._helper_list(uri, query)
 
     def list_meters(self, query=None):
         uri = '%s/meters' % self.uri_prefix
-        return self.helper_list(uri, query)
+        return self._helper_list(uri, query)
 
     def list_alarms(self, query=None):
         uri = '%s/alarms' % self.uri_prefix
-        return self.helper_list(uri, query)
+        return self._helper_list(uri, query)
 
     def list_statistics(self, meter, period=None, query=None):
         uri = "%s/meters/%s/statistics" % (self.uri_prefix, meter)
-        return self.helper_list(uri, query, period)
+        return self._helper_list(uri, query, period)
 
     def list_samples(self, meter_id, query=None):
         uri = '%s/meters/%s' % (self.uri_prefix, meter_id)
-        return self.helper_list(uri, query)
+        return self._helper_list(uri, query)
 
     def get_resource(self, resource_id):
         uri = '%s/resources/%s' % (self.uri_prefix, resource_id)