Update tox.ini for python3

Update tox.ini for python3

Change-Id: I87436c111b415f33d82829bbd19ccac38fdf6f2b
Co-Authored-By: Dharmendra Kushwaha<dharmendra.kushwaha@india.nec.com>

.zuul.yaml

@@ -5,6 +5,7 @@
       - openstack-python-jobs
       - openstack-python35-jobs
       - openstack-python36-jobs
+      - openstack-python37-jobs
       - publish-openstack-docs-pti
     check:
       jobs:

tox.ini

@@ -1,10 +1,10 @@
 [tox]
-envlist = py36,py27,pep8,docs
+envlist = py37,py36,py27,pep8,docs
 minversion = 2.0
 skipsdist = True
 
 [testenv]
-basepython=python2.7
+basepython = python3
 usedevelop = True
 setenv =
   VIRTUAL_ENV={envdir}
@@ -18,10 +18,12 @@ commands =
   {envpython} {toxinidir}/manage.py test tacker_horizon --settings=tacker_horizon.test.settings {posargs}
 
 [testenv:py27]
+basepython = python2.7
 commands =
   {envpython} {toxinidir}/manage.py test tacker_horizon --settings=tacker_horizon.test.settings {posargs}
 
 [testenv:py36]
+basepython = python3.6
 commands =
   {envpython} {toxinidir}/manage.py test tacker_horizon --settings=tacker_horizon.test.settings {posargs}