Use conductor entity class constant instead of raw string

Now that the conductor base class provides a constant that
all entity objects produced from it should provide we can
now use that to ensure we are only registering a limited set
of entity kinds.

Change-Id: I0e71a8da64a228fdc68c9de941aaccf54d493d96

taskflow/jobs/backends/impl_zookeeper.py

@@ -30,6 +30,7 @@ from oslo_utils import timeutils
 from oslo_utils import uuidutils
 import six
 
+from taskflow.conductors import base as c_base
 from taskflow import exceptions as excp
 from taskflow.jobs import base
 from taskflow import logging
@@ -566,7 +567,7 @@ class ZookeeperJobBoard(base.NotifyingJobBoard):
 
     def register_entity(self, entity):
         entity_type = entity.kind
-        if entity_type == 'conductor':
+        if entity_type == c_base.Conductor.ENTITY_KIND:
             entity_path = k_paths.join(self.entity_path, entity_type)
             self._client.ensure_path(entity_path)