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.
Change-Id: I61e683ab0119e4ae90b7107f0690528d789e3875
f4c0468b36