CI: Fix Debian aarch64 jobs

Debian defaults to Python2 which is not complete in aarch64 images.
This patch changes CI to always use Python3.

We need to install several Python modules to have working ussuri jobs.

"Failed to import the required Python library (setuptools) on primary's Python /usr/bin/python3."

And then several Python2 ones for train->ussuri upgrade jobs:

"Unable to find any of pip2 to use. pip needs to be installed."

Change-Id: Ia0d3ff15d97d1cabbb0b8e7f32e8712ca3f94732

tests/pre.yml

@@ -25,11 +25,39 @@
   tasks:
     # NOTE(yoctozepto): we use gawk to add time to each logged line
     # outside of Ansible (e.g. for init-runonce)
-    - name: Install gawk
+    - name: Install gawk and Python modules
       package:
-        name: gawk
+        name:
+          - gawk
+          - python3-pip
+          - python3-setuptools
+          - python3-virtualenv
+          - python3-wheel
       become: true
 
+    # FIXME(hrw): drop when previous_release == ussuri
+    - name: Install Python2 modules for stable/train
+      package:
+        name:
+          - python-pip
+          - python-setuptools
+          - python-virtualenv
+          - python-wheel
+      become: true
+      when:
+        - is_upgrade
+        - previous_release == 'train'
+        - not (ansible_os_family == "RedHat" and ansible_distribution_major_version | int == 8)
+
+    # NOTE(hrw): On RedHat systems it is part of python3-virtualenv
+    - name: Install virtualenv on Debian systems
+      package:
+        name:
+          - virtualenv
+      become: true
+      when:
+        ansible_os_family == "Debian"
+
     - name: Ensure /tmp/logs/ dir
       file:
         path: "{{ logs_dir }}"

zuul.d/base.yaml

@@ -38,6 +38,9 @@
       neutron_external_network_prefix_length: "24"
       neutron_external_interface_name: vxlan1
       tls_enabled: false
+      # NOTE(yoctozepto): Ansible on Debian defaults to /usr/bin/python which is
+      # python2. Let's use python3 instead as expected in 2020 and beyond.
+      ansible_python_interpreter: python3
     roles:
       - zuul: zuul/zuul-jobs