Add gcs_service_enabled function to avoid getting environment variable directly (#7742)

python/ray/node.py

@@ -626,7 +626,7 @@ class Node:
         # If this is the head node, start the relevant head node processes.
         self.start_redis()
 
-        if os.environ.get(ray_constants.RAY_GCS_SERVICE_ENABLED, True):
+        if ray_constants.GCS_SERVICE_ENABLED:
             self.start_gcs_server()
         else:
             self.start_raylet_monitor()