[WIP] Update python SDK to use external SDK harness image

[emilymye]

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Post-Commit Tests Status (on master branch)

Lang	SDK	ULR	Dataflow	Flink	Samza	Spark	Twister2
Go		---	---		---		---
Java
Python		---			---		---
XLang		---			---		---

Pre-Commit Tests Status (on master branch)

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

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