Set tests ids file from env variable

tavern_time_request/request.py

 import logging
 import requests
 import csv
+import os
 from tavern.util import exceptions
 from tavern._plugins.rest.request import RestRequest
 from time import perf_counter as perf
@@ -13,10 +14,12 @@ class HivemindRequest(RestRequest):
         try:
             start_time = perf()
             test_id = uuid4()
-            with open("/tmp/test_ids.csv", "a") as f:
-                writer = csv.writer(f)
-                writer.writerow([str(test_id), start_time])
-            self._request_args['json']['id'] = str(test_id)
+            file_name = os.getenv("HIVEMIND_BENCHMARKS_IDS_FILE", None)
+            if file_name is not None:
+                with open(file_name, "a") as f:
+                    writer = csv.writer(f)
+                    writer.writerow([str(test_id), start_time])
+                self._request_args['json']['id'] = str(test_id)
             return self._prepared()
         except requests.exceptions.RequestException as e:
             logger.exception("Error running prepared request")