MT#56231 workaround for django-celery-beat issue, take three

* fix args for gerrit.tasks.cleanup
* fix flower links using proper url_prefix in config

Change-Id: I2665e2cc4bd361e086e5c65f38700c3d0d84380d

repoapi/settings/prod.py

@@ -164,10 +164,11 @@ CELERY_BEAT_SCHEDULE = {
     "gerrit-cleanup": {
         "task": "gerrit.tasks.cleanup",
         "schedule": crontab(hour=7, minute=30, day_of_month=15),
-        "args": (4),
+        "args": ([4]),
     },
 }
 CELERY_TIMEZONE = "UTC"
+FLOWER_URL_PREFIX = "flower"
 
 JBI_BASEDIR = VAR_DIR / "jbi_files"
 JBI_ARTIFACT_JOBS = [