Propagate SWAN_DEV env variable to culler service

The culler service invokes check_ticket.sh, which invokes
eos_token.sh, which makes use of the SWAN_DEV variable to decide
on its behaviour.

Without this setting, the culler service does not get such
variable from the hub, even when it's set for a dev cluster.

swan/files/swan_config.py

@@ -129,6 +129,7 @@ def swan_pod_hook(spawner, pod):
             "name": "cull-idle",
             "admin": True,
             "command": cull_cmd,
+            "environment": {'SWAN_DEV': os.environ.get('SWAN_DEV', 'false')}
         }
     )