Use Python 2 on CentOS 7

Python 3, CentOS 7 and SELinux do not mix well.

Change-Id: I8a179aad45b89d7eee0716abd5e7428dc51e8b11

playbooks/tenks-deploy-teardown/pre.yml

@@ -23,12 +23,13 @@
       become: true
       package:
         name: python3
+      when: ansible_python.version.major == 3
 
     - name: Create virtualenv for tenks
       pip:
         requirements: "{{ tenks_src_dir }}/requirements.txt"
         virtualenv: "{{ tenks_venv }}"
-        virtualenv_python: python3
+        virtualenv_python: "{{ ansible_python.executable }}"
 
     - name: Template requirements overrides
       template:

playbooks/tenks-deploy-teardown/templates/tenks-overrides.yml.j2

@@ -1,6 +1,6 @@
 ---
 # Use python3 on Ubuntu remote hosts.
-ansible_python_interpreter: "{{ '/usr/bin/python3' if ansible_os_family != 'RedHat' else '/usr/bin/python2' }}"
+ansible_python_interpreter: "{{ '/usr/bin/python3' if ansible_python.version.major == 3 else '/usr/bin/python2' }}"
 
 # This file holds the config given to Tenks when running the zuul job,
 # tenks-deploy-teardown. It assumes the existence of the bridge `breth1`.