Bump tox min version to 2.3.1

We will use passenv in the file once fullstack cases
are up. For that we can bump tox min version to 2.3.1
like other projects.

Change-Id: I42a824c1878dda82ea8197469d061c491b4080db

tox.ini

@@ -1,5 +1,5 @@
 [tox]
-minversion = 1.6
+minversion = 2.3.1
 envlist = py27,py35,pep8
 skipsdist = True
 
@@ -20,6 +20,7 @@ commands = find {toxinidir} -type f -name "*.py[c|o]" -delete
 [testenv:fullstack]
 basepython = python2.7
 setenv = OS_TEST_PATH=./kuryr/tests/fullstack
+passenv = OS_*
 
 [testenv:debug]
 commands = oslo_debug_helper {posargs}