[BEAM-9322] Fix tag output names within Dataflow to be consistent with values used in proto.

[lukecwik]

This updates the Python SDK to drop 'out'_ and 'out' mangling in favor of using 'None' as the main output when there are more then one output defined otherwise the only output becomes the main output.

---

Thank you for your contribution! Follow this checklist to help us incorporate your contribution quickly and easily:

• Choose reviewer(s) and mention them in a comment (R: @username).
• Format the pull request title like [BEAM-XXX] Fixes bug in ApproximateQuantiles, where you replace BEAM-XXX with the appropriate JIRA issue, if applicable. This will automatically link the pull request to the issue.
• Update CHANGES.md with noteworthy changes.
• If this contribution is large, please file an Apache Individual Contributor License Agreement.

See the Contributor Guide for more tips on how to make review process smoother.

Post-Commit Tests Status (on master branch)

Lang	SDK	Apex	Dataflow	Flink	Gearpump	Samza	Spark
Go		---	---		---	---
Java
Python		---			---	---
XLang	---	---	---		---	---

Pre-Commit Tests Status (on master branch)

---	Java	Python	Go	Website
Non-portable
Portable	---		---	---

See .test-infra/jenkins/README for trigger phrase, status and link of all Jenkins jobs.

[lukecwik]

R: @ananvay @rohdesamuel
CC: @robertwb @aaltay

[ananvay]

Thanks a lot Luke! Overall LGTM, assuming the tests pass.

[rohdesamuel]

Thanks for the cleanup! This is a lot cleaner.