[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

(cherry picked from commit 23f5bdf06a388e08ea5a69e848f0ecd5165aa481)
Signed-off-by: Josh Rosen <joshrosen@databricks.com>
[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

(cherry picked from commit b9e1c2eb9b6f7fb609718ef20048a8da452d881b)
Signed-off-by: Reynold Xin <rxin@databricks.com>