[SPARK-2140] Updating heap memory calculation for YARN stable and alpha.

Updated pull request, reflecting YARN stable and alpha states. I am getting intermittent test failures on my own test infrastructure. Is that tracked anywhere yet?

Author: Chris Cope <ccope@resilientscience.com>

Closes #2253 from copester/master and squashes the following commits:

5ad89da [Chris Cope] [SPARK-2140] Removing calculateAMMemory functions since they are no longer needed.
52b4e45 [Chris Cope] [SPARK-2140] Updating heap memory calculation for YARN stable and alpha.