build containers: Allow setting of Python version used.

Currently the pipeline used to create test containers is using Python
3.6.15, and Ansible 2.12 requires, at least, Python 3.8.

This change adds a new parameter to build container template,
`python_version`, which is set by default to '3.x', meaning it will use
the latest Python version available (for version 3) if the parameter is
not explicitly set.

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