Migrate from testr to stestr

* 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
diff --git a/test-requirements.txt b/test-requirements.txt
index be225d8..23614af 100644
--- a/test-requirements.txt
+++ b/test-requirements.txt
@@ -8,3 +8,4 @@
 # hacking to 3.2.0 or above. hacking 3.2.0 takes
 # care of pyflakes version compatibilty.
 pyflakes>=2.1.1
+stestr>=2.0.0 # Apache-2.0