[SPARK-14505][CORE] Fix bug : creating two SparkContext objects in the same...

[SPARK-14505][CORE] Fix bug : creating two SparkContext objects in the same jvm, the first one will can not run any task!

After creating two SparkContext objects in the same jvm(the second one can not be created successfully!),
use the first one to run job will throw exception like below:

![image](https://cloud.githubusercontent.com/assets/7162889/14402832/0c8da2a6-fe73-11e5-8aba-68ee3ddaf605.png)

Author: Allen <yufan_1990@163.com>

Closes #12273 from the-sea/context-create-bug.