[FLINK-20469][minicluster] Enable TaskManager start and terminate in MiniCluster

[azagrebin]

Currently we expose startTaskManager/terminateTaskManager only in internal TestingMiniCluster.
Nonetheless, they are useful methods to implement IT cases similar to E2E tests.
The PR exposes them in MiniCluster documenting the TM indexing.

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[tillrohrmann]

Thanks for creating this PR @azagrebin. LGTM. +1 for merging.