Revert "Remove --flink-master-url alias."

[robertwb]

This reverts commit 1ccecd3.

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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	---		---	---

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[tweise]

The alias is needed. Looks like we are spinning wheels. Please allow for appropriate review.

[tweise]

Run Python2_PVR_Flink PreCommit

[tweise]

Merging this the alias is required for backward compatibility.

[mxm]

For reference: We just renamed flink-master-url to flink-master for consistency, but we made flink-master-url an alias to ensure backwards-compatibility, because this parameter has been used for more than a year. See also: https://github.com/apache/beam/pull/9844/files#diff-e52a25378b181af8cecbf3d274946fd0R43

[robertwb]

Yes. This PR puts things back into the state they were. Looks like we're stuck with the alias.