[FLINK-18507][python] Move get_config implementation to TableEnvironment

[dianfu]

What is the purpose of the change

This pull request moves get_config implementation to TableEnvironment to eliminate the code duplication.

Brief change log

• Move get_config implementation to TableEnvironment

Verifying this change

This change is a code cleanup without any test coverage.

Does this pull request potentially affect one of the following parts:

• Dependencies (does it add or upgrade a dependency): (no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (no)
• The serializers: (no)
• The runtime per-record code paths (performance sensitive): (no)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Kubernetes/Yarn/Mesos, ZooKeeper: (no)
• The S3 file system connector: (no)

Documentation

• Does this pull request introduce a new feature? (no)
• If yes, how is the feature documented? (not applicable)

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Automated Checks

Last check on commit 071fcd5 (Tue Jul 07 02:38:14 UTC 2020)

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• ❓ 1. The [description] looks good.
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• ❓ 5. Overall code [quality] is good.

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CI report:

• 071fcd5 Azure: SUCCESS

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[WeiZhong94]

LGTM, +1 to merge.