CI: Use python3 in deploy-teardown jobs

This change modifies the deploy-teardown jobs to use python 3 in the
local virtualenv in which ansible is installed. It also switches to use
python 3  on Ubuntu remote hosts via ansible_python_interpreter. Python
3 support for CentOS remote hosts will be added with CentOS 8 support.

Depends-On: https://github.com/stackhpc/ansible-role-libvirt-host/pull/19
Change-Id: I1e117525a49c1732c70a3f5925b1b88a3a865e11

playbooks/tenks-deploy-teardown/pre.yml

@@ -19,10 +19,16 @@
         src: tenks-overrides.yml.j2
         dest: "{{ tenks_overrides_path }}"
 
+    - name: Install python3
+      become: true
+      package:
+        name: python3
+
     - name: Create virtualenv for tenks
       pip:
         requirements: "{{ tenks_src_dir }}/requirements.txt"
         virtualenv: "{{ tenks_venv }}"
+        virtualenv_python: python3
 
     - name: Template requirements overrides
       template:

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

@@ -1,4 +1,7 @@
 ---
+# Use python3 on Ubuntu remote hosts.
+ansible_python_interpreter: "{{ '/usr/bin/python3' if ansible_os_family != 'RedHat' 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`.