Add Python3 project templates for Train release

Following the process set down in the TC resolution on updating Python
versions:

https://governance.openstack.org/tc/resolutions/20181024-python-update-process.html

this change adds the Zuul template for Python 3 unit test jobs in the
Train release. The TC has determined that:

* The latest released version of Python that is available for testing is
  3.7.
* The only version of Python 3 that is the default in the
  distributions listed in the PTI (CentOS 7, Ubuntu 18.04 Bionic, openSUSE
  Leap 15) is Python 3.6. (Python 2.7 is the default in CentOS 7.)
* All Xenial-based integration tests will be migrated to Bionic before
  the Train cycle begins, so there is no need to test Python 3.5.

Therefore the Python 3 versions unit tested in Train will be Python 3.6
and Python 3.7. (Python 2.7 will also be tested.)

http://eavesdrop.openstack.org/meetings/tc_python3/2019/tc_python3.2019-03-07-21.00.html

Change-Id: I8b5fc61a6e1c192efed22bfd02155e22187464c1

zuul.d/project-templates.yaml

@@ -413,6 +413,92 @@
             required-projects:
               - openstack/neutron
 
+- project-template:
+    name: openstack-python3-train-jobs
+    description: |
+      Runs unit tests for an OpenStack Python project under the CPython
+      version 3 releases designated for testing in the Train release.
+    check:
+      jobs:
+        - openstack-tox-py36
+        - openstack-tox-py37
+    gate:
+      jobs:
+        - openstack-tox-py36
+        - openstack-tox-py37
+
+- project-template:
+    name: openstack-python3-train-jobs-horizon
+    description: |
+      The standard :zuul:project_template:`openstack-python3-train-jobs`
+      but with horizon added as a required-projects. This checks out
+      the project code and allows tox_install_siblings to use horizon
+      when building the venv.
+    check:
+      jobs:
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/horizon
+        - openstack-tox-py37:
+            required-projects:
+              - openstack/horizon
+    gate:
+      jobs:
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/horizon
+        - openstack-tox-py37:
+            required-projects:
+              - openstack/horizon
+
+- project-template:
+    name: openstack-python3-train-jobs-neutron
+    description: |
+      The standard :zuul:project_template:`openstack-python3-train-jobs`
+      but with neutron added as a required-projects. This checks out
+      the project code and allows tox_install_siblings to use neutron
+      when building the venv.
+    check:
+      jobs:
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/neutron
+        - openstack-tox-py37:
+            required-projects:
+              - openstack/neutron
+    gate:
+      jobs:
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/neutron
+        - openstack-tox-py37:
+            required-projects:
+              - openstack/neutron
+
+- project-template:
+    name: openstack-python3-train-jobs-ceilometer
+    description: |
+      The standard :zuul:project_template:`openstack-python3-train-jobs`
+      but with ceilometer added as a required-projects. This checks out
+      the project code and allows tox_install_siblings to use ceilometer
+      when building the venv.
+    check:
+      jobs:
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/ceilometer
+        - openstack-tox-py37:
+            required-projects:
+              - openstack/ceilometer
+    gate:
+      jobs:
+        - openstack-tox-py36:
+            required-projects:
+              - openstack/ceilometer
+        - openstack-tox-py37:
+            required-projects:
+              - openstack/ceilometer
+
 - project-template:
     name: openstack-python35-jobs
     description: |