Issues #562,#561,#560:

- No source of entropy in _get_nearest_mirrors_by_network_data()
- _get_nearest_mirrors_by_network_data() fails to exclude near-by private mirrors for extra options.
- Exclude the private mirrors from the mirrors list in the case of fallback behavior

- The Azure mirrors have allowed list of arches
- Decrease level of logging messages in some cases
- Cache subnets of Azure/AWS cloud

src/backend/api/utils.py

@@ -255,7 +255,7 @@ async def get_azure_subnets(http_session: ClientSession):
             properties = value['properties']
             subnets[properties['region'].lower()] = \
                 properties['addressPrefixes']
-    await set_subnets_to_cache('aws_subnets', subnets)
+    await set_subnets_to_cache('azure_subnets', subnets)
     return subnets