[FLINK-7653] Properly implement Dispatcher#requestClusterOverview

[tillrohrmann]

What is the purpose of the change

This commit implements the ClusterOverview generation on the Dispatcher. In
order to do this, the Dispatcher requests the ResourceOverview from the
ResourceManager and the job status from all JobMasters. After receiving all
information, it is compiled into the ClusterOverview.

Note: StatusOverview has been renamed to ClusterOverview

Verifying this change

Tested the changes manually.

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)

[zentol]

+1

[tillrohrmann]

Thanks for the review @zentol. Rebasing onto the latest master and merging after Travis gives green light.