Add indicator prefix to usafacts archiver params

Required, currently manually provided in https://github.com/cmu-delphi/covidcast-indicators/blob/0943e38f4a7e5a33d521f0f807313059f4c8e300/usafacts/delphi_usafacts/run.py#L99 needed to be provided to avoid https://github.com/cmu-delphi/covidcast-indicators/blob/f83033786a912c475e53654a2bc58c2d067e2727/_delphi_utils_python/delphi_utils/archive.py#L143

Author: qx-teo

ansible/templates/usafacts-params-prod.json.j2

@@ -13,6 +13,7 @@
       "aws_secret_access_key": "{{ delphi_aws_secret_access_key }}"
     },
     "bucket_name": "delphi-covidcast-indicator-output",
+    "indicator_prefix": "usafacts",
     "cache_dir": "./cache"
   },
   "validation": {

usafacts/params.json.template

@@ -14,6 +14,7 @@
       "aws_secret_access_key": ""
     },
     "bucket_name": "",
+    "indicator_prefix": "usafacts",
     "cache_dir": "./cache"
   },
   "validation": {