pep8/flake8 checks for system tests

Currently the python code for system tests doesn't have an checks on style. Similar to checkstyle in java/scala, we should be checking the style for Python code. These consists of at least 2 parts:

• Add the configs that allow you to run these checks
• Get the Kafka CI systems to run these as a regular part of the build process, in addition to building & testing the java/scala code.

The latter is hard to estimate the effort for since it depends on what is available on the Apache Jenkins slaves that we build on. Hopefully at least python + pip would be there such that augmenting the Jenkins build to also run flake8 wouldn't be too tough.

If it makes sense while doing this, we might also want to get Python3 compatibility and even just move to it being a requirement. Whether this is a requirement for this JIRA might depend on the Apache jenkins environments and what python versions they have available.