[FLINK-13193][tests] Enable custom flink cluster config per test

[GJL]

What is the purpose of the change

Enable users to set a cluster configuration in the test spec which will have precedence over the default configuration.

Brief change log

• See commits

Verifying this change

This change added tests and can be verified as follows:

• Added unit test to verify that default Flink configuration can be overriden

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

• Dependencies (does it add or upgrade a dependency): (yes / no)
• The public API, i.e., is any changed class annotated with @Public(Evolving): (yes / no)
• The serializers: (yes / no / don't know)
• The runtime per-record code paths (performance sensitive): (yes / no / don't know)
• Anything that affects deployment or recovery: JobManager (and its components), Checkpointing, Yarn/Mesos, ZooKeeper: (yes / no / don't know)
• The S3 file system connector: (yes / no / don't know)

Documentation

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

cc: @tillrohrmann

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[flinkbot]

CI report for commit 8e7598b: SUCCESS Build

[tillrohrmann]

LGTM. +1 for merging into master and release-1.9 in order to enable Jepsen tests with fine grained recovery if this is not yet running.

[flinkbot]

CI report:

• 8e7598b : SUCCESS Build