more refactor and test things

brian_polling_manager/interface_stats/brian.py

brian_polling_manager/interface_stats/cli.py

brian_polling_manager/interface_stats/config.py

brian_polling_manager/interface_stats/errors.py

brian_polling_manager/interface_stats/vendors/__init__.py

brian_polling_manager/interface_stats/vendors/common.py

brian_polling_manager/interface_stats/vendors/juniper.py

brian_polling_manager/interface_stats/vendors/nokia.py

test/interface_stats/test_interface_stats.py

test/interface_stats/test_interface_stats_e2e.py