Pin neutron, keystone and cinder tempest plugins to py2 compatible versions where needed

On the OSA Train branch, Centos-7 deploys with python2 and the other
operating systems use python3. Debian stable (stretch) uses python
3.5.

This patch pins the tempest plugin versions to ones which support
python2.7 and 3.5 or later.

Depends-On: https://review.opendev.org/780562
Change-Id: I4c9793aaeca46e2e0b7528de8fa0dc222803debb

vars/main.yml

@@ -59,7 +59,7 @@ _tempest_plugins:
   cinder:
     - name: cinder-tempest-plugin
       repo: https://opendev.org/openstack/cinder-tempest-plugin
-      branch: master
+      branch: 0.3.1
       install: "{{ tempest_service_available_cinder | bool }}"
   cloudkitty:
     - name: cloudkitty-tempest-plugin
@@ -89,7 +89,7 @@ _tempest_plugins:
   keystone:
     - name: keystone-tempest-plugin
       repo: https://opendev.org/openstack/keystone-tempest-plugin
-      branch: master
+      branch: 0.3.0
       install: yes
   magnum:
     - name: magnum-tempest-plugin
@@ -124,7 +124,7 @@ _tempest_plugins:
   neutron:
     - name: neutron-tempest-plugin
       repo: https://opendev.org/openstack/neutron-tempest-plugin
-      branch: 0.9.0
+      branch: "{{ (tempest_venv_python_executable == 'python2') | ternary('0.2.0', '0.3.0') }}"
       install: "{{ tempest_service_available_neutron | bool }}"
   novajoin:
     - name: novajoin-tempest-plugin