Add .zuul.yaml file with tox job

Move legacy tox job in-tree and convert to Zuul v3 native syntax.

Use tox as parent, we do not need constraints setup.

Name the job "publish" instead of previously validate since it calls
the publish environment.

Change-Id: Ib676370e4ccf6c6a0c2f9384a0e023b54521a3b7

.zuul.yaml

@@ -0,0 +1,18 @@
+- project:
+    check:
+      jobs:
+        - service-types-authority-tox-publish
+    gate:
+      jobs:
+        - service-types-authority-tox-publish
+
+
+- job:
+    name: service-types-authority-tox-publish
+    parent: tox
+    description: |
+      Run tests for service-types-authority project.
+
+      Uses tox with the ``publish`` environment.
+    vars:
+      tox_envlist: publish