Add ability to exclude a specific platform

The ensure-podman role will never work on focal, so we need to
exclude that without hindering our ability to manage the rest of
the platform set.

Change-Id: I3eadb9450c9d1b8bf8cb58b4ea5288c48f647ac1

tools/update-test-platforms.py

@@ -110,6 +110,9 @@ def handle_file(fn):
             platforms = sorted(platforms)
             if platforms:
                 for platform in platforms:
+                    exclude_tag = f'exclude-{platform}'
+                    if exclude_tag in tags:
+                        continue
                     voting = False if platform in NON_VOTING else True
                     ojob = CommentedMap()
                     ojob['name'] = job['name'] + '-' + platform