Set ensure_pip_virtualenv_command in Focal proposal jobs

Previously we had this error:

  No key/value pairs provided, at least one is required for this action to succeed

Which appears to be due to the default(omit) condition when setting
ensure_pip_virtualenv_command in focal proposal jobs. The issue is
default(omit) doesn't do what you think it does when defining a
variable. It is really only useful for defining module parameters that
may be omitted.

Instead we need to call ensure-tox two different ways depending on
whether or not we want to override the defaults for
ensure_pip_virtualenv_command. One method for Bionic and another for
Focal keyed off of whether or not _venv_command is defined.

Change-Id: I0cbca64f4a31c8b4eacb5e1c50f2e9fb289ce18e

playbooks/proposal/pre.yaml

@@ -76,9 +76,17 @@
           vars:
             python_version: 3.9
 
-    - name: Install tox
+    - name: Install tox with venv command override
       include_role:
         name: ensure-tox
       vars:
         ensure_global_symlinks: true
-        ensure_pip_virtualenv_command: '{{ _venv_command | default(omit) }}'
+        ensure_pip_virtualenv_command: '{{ _venv_command }}'
+      when: _venv_command is defined
+
+    - name: Install tox without venv command override
+      include_role:
+        name: ensure-tox
+      vars:
+        ensure_global_symlinks: true
+      when: _venv_command is not defined