…on registered cores rather than accepted cores

See JIRA

Unit tests, Mesos/Spark integration tests

cc skonto susanxhuynh

Author: Michael Gummelt <mgummeltmesosphere.io>

Closes #17045 from mgummelt/SPARK-19373-registered-resources.

## What changes were proposed in this pull request?

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## How was this patch tested?

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Please review http://spark.apache.org/contributing.html before opening a pull request.

Author: Michael Gummelt <mgummelt@mesosphere.io>

Closes #17129 from mgummelt/SPARK-19373-registered-resources-2.1.