- Defined executorCores from "spark.mesos.executor.cores"
- Changed the amount of mesosExecutor's cores to executorCores.
- Added new configuration option on running-on-mesos.md

Author: Jongyoul Lee <jongyoul@gmail.com>

Closes #5063 from jongyoul/SPARK-6350 and squashes the following commits:

9238d6e [Jongyoul Lee] [SPARK-6350][Mesos] Make mesosExecutorCores configurable in mesos "fine-grained" mode - Fixed docs - Changed configuration name - Made mesosExecutorCores private
2d41241 [Jongyoul Lee] [SPARK-6350][Mesos] Make mesosExecutorCores configurable in mesos "fine-grained" mode - Fixed docs
89edb4f [Jongyoul Lee] [SPARK-6350][Mesos] Make mesosExecutorCores configurable in mesos "fine-grained" mode - Fixed docs
8ba7694 [Jongyoul Lee] [SPARK-6350][Mesos] Make mesosExecutorCores configurable in mesos "fine-grained" mode - Fixed docs
7549314 [Jongyoul Lee] [SPARK-6453][Mesos] Some Mesos*Suite have a different package with their classes - Fixed docs
4ae7b0c [Jongyoul Lee] [SPARK-6453][Mesos] Some Mesos*Suite have a different package with their classes - Removed TODO
c27efce [Jongyoul Lee] [SPARK-6453][Mesos] Some Mesos*Suite have a different package with their classes - Fixed Mesos*Suite for supporting integer WorkerOffers - Fixed Documentation
1fe4c03 [Jongyoul Lee] [SPARK-6453][Mesos] Some Mesos*Suite have a different package with their classes - Change available resources of cpus to integer value beacuse WorkerOffer support the amount cpus as integer value
5f3767e [Jongyoul Lee] Revert "[SPARK-6350][Mesos] Make mesosExecutorCores configurable in mesos "fine-grained" mode"
4b7c69e [Jongyoul Lee] [SPARK-6350][Mesos] Make mesosExecutorCores configurable in mesos "fine-grained" mode - Changed configruation name and description from "spark.mesos.executor.cores" to "spark.executor.frameworkCores"
0556792 [Jongyoul Lee] [SPARK-6350][Mesos] Make mesosExecutorCores configurable in mesos "fine-grained" mode - Defined executorCores from "spark.mesos.executor.cores" - Changed the amount of mesosExecutor's cores to executorCores. - Added new configuration option on running-on-mesos.md