upstream CI: Update Python version when building containers.

Newer Ansible versions will require at least Python 3.8 to be used,
and the build containers pipeline was requiring Python 3.6, which is
EOL.

This patch requests the latest Python version available for the
controller, and allows it to be configured to a specific version if,
and when, needed.

tests/azure/templates/build_container.yml

@@ -6,6 +6,9 @@ parameters:
     type: string
   - name: build_scenario_name
     type: string
+  - name: python_version
+    type: string
+    default: 3.x
 
 jobs:
 - job: BuildTestImage${{ parameters.job_name_suffix }}
@@ -13,7 +16,7 @@ jobs:
   steps:
   - task: UsePythonVersion@0
     inputs:
-      versionSpec: '3.6'
+      versionSpec: '${{ parameters.python_version }}'
 
   - script: python -m pip install --upgrade pip setuptools wheel ansible
     displayName: Install tools