Flake8 extensions for specific tempest style checks

We recently changed to flake8 for style checking, however there were a number of specific checks that we put in place for tempest, like the skip bug checks (https://github.com/openstack/tempest/blob/87a52b7dc1bcd8c8d883b515e1fed8e901a92c04/tools/hacking.py#L328)

We need to port this to flake8, and figure out how we carry local flake8 extensions in the tempest tree. This will require syncing with Monty Taylor who did the hacking => flake8 porting.