- Fixed calculateTotalMemory to use spark.mesos.executor.memoryOverhead
- Added testCase

Author: Jongyoul Lee <jongyoul@gmail.com>

Closes #5099 from jongyoul/SPARK-6423 and squashes the following commits:

6747fce [Jongyoul Lee] [SPARK-6423][Mesos] MemoryUtils should use memoryOverhead if it's set - Changed a description of spark.mesos.executor.memoryOverhead
475a7c8 [Jongyoul Lee] [SPARK-6423][Mesos] MemoryUtils should use memoryOverhead if it's set - Fit the import rules
453c5a2 [Jongyoul Lee] [SPARK-6423][Mesos] MemoryUtils should use memoryOverhead if it's set - Fixed calculateTotalMemory to use spark.mesos.executor.memoryOverhead - Added testCase