load data files: do not try to download data files when missing

As the leapp upgrade data files are nowadays part of the install rpm,
there is no need to download them anymore. Also, we do not have any
plans to provide the updated data files anywhere out of the rpm.

From that point, do not try to download provided data files anymore.

repos/system_upgrade/common/libraries/fetch.py

@@ -20,6 +20,7 @@ def _get_hint():
     rpmname = 'leapp-upgrade-el{}toel{}'.format(get_source_major_version(), get_target_major_version())
     hint = (
         'All official data files are nowadays part of the installed rpms.'
+        ' That is the only official resource of actual official data files for in-place upgrades.'
         ' This issue is usually encountered when the data files are incorrectly customized, replaced, or removed'
         ' (e.g. by custom scripts).'
         ' In case you want to recover the original file, remove it (if still exists)'
@@ -33,7 +34,9 @@ def _raise_error(local_path, details):
     """
     If the file acquisition fails in any way, throw an informative error to stop the actor.
     """
+    rpmname = 'leapp-upgrade-el{}toel{}'.format(get_source_major_version(), get_target_major_version())
     summary = 'Data file {lp} is missing or invalid.'.format(lp=local_path)
+
     raise StopActorExecutionError(summary, details={'details': details, 'hint': _get_hint()})
 
 
@@ -174,7 +177,7 @@ def load_data_asset(actor_requesting_asset,
 
     try:
         # The asset family ID has the form (major, minor), include only `major` in the URL
-        raw_asset_contents = read_or_fetch(asset_filename, data_stream=data_stream_major)
+        raw_asset_contents = read_or_fetch(asset_filename, data_stream=data_stream_major, allow_download=False)
         asset_contents = json.loads(raw_asset_contents)
     except ValueError:
         msg = 'The {0} file (at {1}) does not contain a valid JSON object.'.format(asset_fulltext_name, asset_filename)