[BP-1.5][FLINK-10637] Start MiniCluster with random REST port

[tillrohrmann]

What is the purpose of the change

This commit starts the MiniCluster used in tests with a random REST port. This
will fix the problem of concurrently executed test cases which fail due to
port conflicts.

This is a backport of #6799 for release-1.5.

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)
• 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)