[BEAM-8430] Change py default sdk_worker_parallelism to 1

[ibzib]

This aligns the Python SDK with the Java SDK.

R: @tweise @mxm @angoenka

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.

[mxm]

Run Python2_PVR_Flink PreCommit

[tweise]

The mismatch actually goes back to f3623e8#diff-525d5d65bedd7ea5e6fce6e4cd57e153 and was masked by the job server option that got removed now.

Please also add a comment to PortableOptions that they need to remain in sync with the Java version.

Longer term it would be nice to define such options in proto.

[ibzib]

@tweise I added a comment.

I agree that seems like a lot of extra work to manually maintain consistency between SDKs. It seems like the current options setup was designed to prioritize flexibility over structure.

[mxm]

Since we already have the option retrieval, we could leverage it to display the supported options from the Java SDK. However, it would be even better to have a language-agnostic option format which could be used by all SDKs to read from a single point of definition.