[SPARK-4373][MLLIB] fix MLlib maven tests

We want to make sure there is at most one spark context inside the same jvm. JoshRosen

Author: Xiangrui Meng <meng@databricks.com>

Closes #3235 from mengxr/SPARK-4373 and squashes the following commits:

6574b69 [Xiangrui Meng] rename LocalSparkContext to MLlibTestSparkContext
913d48d [Xiangrui Meng] make sure there is at most one spark context inside the same jvm
[SPARK-4370] [Core] Limit number of Netty cores based on executor size

Author: Aaron Davidson <aaron@databricks.com>

Closes #3155 from aarondav/conf and squashes the following commits:

7045e77 [Aaron Davidson] Add mesos comment
4770f6e [Aaron Davidson] [SPARK-4370] [Core] Limit number of Netty cores based on executor size