Only install python3.8-venv in translation update jobs

Currently both python3.8 and python3.9 venv package
is being installed, but since python3.9-venv is not
available in bionic, python3.8-venv installation also
fails as mentioned in [1].
This patch moves ensure_pip_from_upstream_interpreters
var from proposal pre playbook to job definition and
override it in translation jobs to attempt installation
only for python3.8-venv.

Also switch back to "python3.8 -m venv" virtualenv_cmd
as with this patch python3.8-venv will get installed.

[1] https://review.opendev.org/c/zuul/zuul-jobs/+/846201
[2] https://review.opendev.org/c/openstack/project-config/+/846390

Change-Id: I9d9a423e3147d3f04c1951a796636e5520202a6d

playbooks/proposal/pre.yaml

@@ -3,11 +3,6 @@
     - name: Ensure pip
       include_role:
         name: ensure-pip
-      vars:
-        ensure_pip_from_packages: True
-        ensure_pip_from_upstream_interpreters:
-          - python3.8
-          - python3.9
 
     - name: Ensure python
       include_role:

zuul.d/jobs.yaml

@@ -834,6 +834,10 @@
         name: ssh_key
     vars:
       python_version: 3.9
+      ensure_pip_from_packages: True
+      ensure_pip_from_upstream_interpreters:
+        - python3.8
+        - python3.9
 
 - job:
     name: propose-update-constraints
@@ -1191,7 +1195,10 @@
       # ubuntu-bionic don't have python 3.9
       python_version: 3.8
       # ubuntu-bionic default python is 3.6 and master upper-constraints are not compatible with it
-      ensure_pip_virtualenv_command: "virtualenv -p python3.8"
+      ensure_pip_virtualenv_command: "/usr/bin/python3.8 -m venv"
+      # Only install python3.8 in ubuntu-bionic
+      ensure_pip_from_upstream_interpreters:
+        - python3.8
       constraints_file: '{{ ansible_user_dir }}/src/opendev.org/openstack/requirements/upper-constraints.txt'
     required-projects:
       - openstack/horizon
@@ -1219,7 +1226,10 @@
       # ubuntu-bionic don't have python 3.9
       python_version: 3.8
       # ubuntu-bionic default python is 3.6 and master upper-constraints are not compatible with it
-      ensure_pip_virtualenv_command: "virtualenv -p python3.8"
+      ensure_pip_virtualenv_command: "/usr/bin/python3.8 -m venv"
+      # Only install python3.8 in ubuntu-bionic
+      ensure_pip_from_upstream_interpreters:
+        - python3.8
       constraints_file: '{{ ansible_user_dir }}/src/opendev.org/openstack/requirements/upper-constraints.txt'
     semaphores: translations
     protected: true