PyUtils: Add atlas-specific flake8 plugins

flake8 is a code linter (style checker) for Python. We use it already as a unit test in a few packages. It's useful to be able to develop ATLAS specific plugins:

• Add infrastructure to develop/install ATLAS-specific flake8 plugins
• Add a first plugin to check for explicit setting of OutputLevel (disabled by default)

There should be no impact of this MR on any existing code as the new plugin will have to be enabled explicitly for the relevant sources/packages.