1. In reviews we usually check import grouping but it is boring.
By using flake8-import-order plugin, we can avoid this.
It enforces loose checking so it sounds good to use it.
This flake8 plugin is already used in tempest.
Note that flake8-import-order version is pinned to avoid unexpected
breakage of pep8 job.
Setup for unit tests of hacking rules is tweaked to disable
flake8-import-order checks. This extension assumes an actual file
exists and causes hacking rule unit tests.
2. This patch is also intend to clean up exceptions to avoid
confusing for other developers and the maintenance-ability as well.
Change-Id: I032892f08e073feb5b822d27d092f041b17d57e1
d8ba5b7a81