foris-controller: update to version 5.5.1

Changed

• tests: Refactor helper functions into shared module.
• tests: Use tox to run tests and linter checks instead of using deprecated setuptools integration for such tasks.
• setup.py: Drop the unnecessary egg suffix in VCS links.
• setup.py: Remove obsolete setuptools keywords (tests_require, setup_requires, dependency_links, ...).
• tox: Use shared tox.ini for foris-controller and modules.

Fixed

• guest: Use correct guest network bridge name for Turris OS 6.0+, so QoS for guest network will be able to start again.
• wan: Do not crash when some of the PPPoE credentials options are missing in uci config.