[Configuration] Add missing py37 and corrected default envlist.

Change order of default tox envlist in order of likeliness to fail on a
new change and add py37 and py36.

Change-Id: If72e480c12385463088e2e0962801cd7095a6f12

tox.ini

@@ -1,7 +1,7 @@
 [tox]
 minversion = 2.0
 skipsdist = True
-envlist = py35,py27,pep8
+envlist = pep8,py37,py36,py35,py27
 
 [testenv]
 usedevelop = True

zuul.d/layout.yaml

@@ -7,6 +7,7 @@
       - openstack-python-jobs
       - openstack-python36-jobs
       - tripleo-standalone-scenarios-full
+      - openstack-python37-jobs
     check:
       jobs:
         - openstack-tox-pep8