[FLINK-7920] Make MiniClusterConfiguration immutable

[tillrohrmann]

What is the purpose of the change

Makes the MiniClusterConfiguration immutable.

Verifying this change

This change is a trivial rework / 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, Yarn/Mesos, ZooKeeper: (no)

Documentation

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

[StephanEwen]

Looks good, +1 to merge this

[tillrohrmann]

Thanks for the review @StephanEwen. Merging this PR.