Add template for charm check and gate

Add new template to cover the required range of py3 versions
that the OpenStack Charms need to be validated against.

This is mainly driven by the Ubuntu versions the charms have
to support which includes back to Ubuntu 16.04 LTS (Python 3.5)
and to the current 20.04 LTS (Python 3.8).

Master branch of all charms will be switched to use this
template in time for the next charm release in August 2020.

Stable branches will not be updated however old templates
have been marked as DEPRECATED FOR REMOVAL.

Change-Id: Ib9ed240aef6680be7c972088ca5337b5740b9c36

zuul.d/project-templates.yaml

@@ -457,6 +457,25 @@
             required-projects:
               - openstack/neutron
 
+- project-template:
+    name: openstack-python3-charm-jobs
+    description: |
+      Runs unit tests for an OpenStack Charm project under the CPython
+      version 3 releases designated for testing across OpenStack
+      releases.
+    check:
+      jobs:
+        - openstack-tox-pep8
+        - openstack-tox-py35
+        - openstack-tox-py36
+        - openstack-tox-py38
+    gate:
+      jobs:
+        - openstack-tox-pep8
+        - openstack-tox-py35
+        - openstack-tox-py36
+        - openstack-tox-py38
+
 - project-template:
     name: openstack-python3-victoria-jobs
     description: |
@@ -1657,7 +1676,8 @@
       Run Python tests for charm projects.
       This run py2.7 job and supposed to be used
       on stable branch until stable/train. Ussuri
-      onwards, py3 template should be used.
+      onwards, py3 template should be used
+      (DEPRECATED FOR REMOVAL).
     check:
       jobs:
         - openstack-tox-pep8
@@ -1683,7 +1703,8 @@
 - project-template:
     name: python35-charm-jobs
     description: |
-      Run tox-pep8 and tox-py35 check and gate jobs.
+      Run tox-pep8 and tox-py35 check and gate jobs
+      (DEPRECATED FOR REMOVAL).
     check:
       jobs:
         - openstack-tox-pep8
@@ -1696,7 +1717,8 @@
 - project-template:
     name: python36-charm-jobs
     description: |
-      Run tox-pep8 and tox-py36 check and gate jobs.
+      Run tox-pep8 and tox-py36 check and gate jobs
+      (DEPRECATED FOR REMOVAL).
     check:
       jobs:
         - openstack-tox-pep8