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