* Replace .testr.conf by .stestr.conf for migration and update
.gitignore and test-requirements.txt files accordingly
* Use py3 as the default runtime for tox and 3.18.0 as the
minversion for tox
Signed-off-by: HeroicHitesh <email.hiteshkumar@gmail.com>
Change-Id: If2662e223ad52c8cae965eb6bdf880c1dce962f2