Ensure python interpreter is installed for tox-py3X

Add tox-py38 and ensure that python interpreter is installed for tox-py3
jobs. The job includes the ensure-python role that checks and installs the
interpreter if python_version is set.

Test py37 to ensure that this works, it's the version currently used for
next OpenStack release, so let's test changes to roles and jobs here.

Run tox-py35 on Xenial, since that Xenial has Python 3.5 - it was run
previously on the default (3.6 in our install) instead, this change
showed that we run the wrong interpreter for tox-py35 on this repo.

Change-Id: I46d6780a5a32c443c37ef040b2af2459641dd123

doc/source/python-jobs.rst

@@ -7,6 +7,7 @@ Python Jobs
 .. zuul:autojob:: tox-py35
 .. zuul:autojob:: tox-py36
 .. zuul:autojob:: tox-py37
+.. zuul:autojob:: tox-py38
 .. zuul:autojob:: tox-docs
 .. zuul:autojob:: tox-linters
 .. zuul:autojob:: tox-molecule

zuul-tests.d/project.yaml

@@ -7,12 +7,16 @@
       jobs:
         - zuul-tox-docs
         - tox-py27
-        - tox-py35
+        - tox-py35:
+            nodeset: ubuntu-xenial
+        - tox-py37
     gate:
       jobs:
         - zuul-tox-docs
         - tox-py27
-        - tox-py35
+        - tox-py35:
+            nodeset: ubuntu-xenial
+        - tox-py37
     promote:
       jobs:
         - zuul-promote-docs

zuul.d/python-jobs.yaml

@@ -66,6 +66,7 @@
       Uses tox with the ``py34`` environment.
     vars:
       tox_envlist: py34
+      python_version: 3.4
 
 - job:
     name: tox-py35
@@ -76,6 +77,7 @@
       Uses tox with the ``py35`` environment.
     vars:
       tox_envlist: py35
+      python_version: 3.5
 
 - job:
     name: tox-py36
@@ -86,6 +88,7 @@
       Uses tox with the ``py36`` environment.
     vars:
       tox_envlist: py36
+      python_version: 3.6
 
 - job:
     name: tox-py37
@@ -96,6 +99,18 @@
       Uses tox with the ``py37`` environment.
     vars:
       tox_envlist: py37
+      python_version: 3.7
+
+- job:
+    name: tox-py38
+    parent: tox
+    description: |
+      Run unit tests for a Python project under cPython version 3.8.
+
+      Uses tox with the ``py38`` environment.
+    vars:
+      tox_envlist: py38
+      python_version: 3.8
 
 - job:
     name: tox-docs