Add non-voting dnspython latest release unit test

This patch adds a non-voting job that runs the Designate unit tests
with the latest release of the dnspython package.

Change-Id: Ief33213ab5cc549b7fa4babc4eabf72aaeb98b6b

.zuul.yaml

@@ -147,6 +147,14 @@
       - ^etc/.*$
       - ^releasenotes/.*$
 
+- job:
+    name: designate-tox-dnspython-latest
+    parent: openstack-tox
+    description: |
+      Run the Designate unit tests with the latest release of dnspython.
+    vars:
+      tox_envlist: dnspython-latest
+
 - project-template:
     name: designate-devstack-jobs
     check:
@@ -183,6 +191,8 @@
     check:
       jobs:
         - neutron-tempest-plugin-designate-scenario
+        - designate-tox-dnspython-latest:
+            voting: false
     gate:
       queue: designate
       jobs:

tox.ini

@@ -189,3 +189,8 @@ deps =
   -c{toxinidir}/lower-constraints.txt
   -r{toxinidir}/test-requirements.txt
   -r{toxinidir}/requirements.txt
+
+[testenv:dnspython-latest]
+commands =
+  python -m pip install -U dnspython --pre
+  {[testenv]commands}