[k2] remove cluster from k2 json logs (#1638)

This PR updates list of K2-specific tags. `"cluster"` tag was added.

Author: Shamzik

tests/python/lib/k2_server.py

@@ -71,6 +71,7 @@ def _process_json_log(self, log_record):
         log_record.pop("file", "")
         log_record.pop("line", "")
         log_record.pop("hostname", "")
+        log_record.pop("cluster", "")
 
         # convert raw string to python dict
         tags = log_record.get("tags")