Add support to exclude a region from pipedream

[mattgauntseo-sentry]

There a couple of cleanup changes in this PR too:

• Use get_fixtures to get the set of jsonnet files to test against
• Use the pipeline name for the filename instead of generating a shorter name (this won't be noticed by folks as we will be removing generated pipelines from repos along with this change thanks to the GoCD Jsonnet plugin now working)