Disable pyXY tests on internal CI

this repo has no actual unit tests and when executing test runner
attempts to run actual tempest tests.

The CI job checks the list of default envs before runnning given
tox env. While this is actually wrong (those pyXY envs are
actually auto-generated by tox), removing them from default env
list unblocks our internal CI.

Change-Id: I782ac77821cc2c84ff3b0c7d6595ac61644330f2
diff --git a/tox.ini b/tox.ini
index 196d873..06f9b47 100644
--- a/tox.ini
+++ b/tox.ini
@@ -1,6 +1,6 @@
 [tox]
 minversion = 2.0
-envlist = py35,py27,flake8
+envlist = pep8
 skipsdist = True
 
 [testenv]