[SPARK-9562] Change reference to amplab/spark-ec2 from mesos/

cc srowen pwendell nchammas

Author: Shivaram Venkataraman <shivaram@cs.berkeley.edu>

Closes #7899 from shivaram/spark-ec2-move and squashes the following commits:

7cc22c9 [Shivaram Venkataraman] Change reference to amplab/spark-ec2 from mesos/

ec2/spark_ec2.py

@@ -90,7 +90,7 @@ DEFAULT_SPARK_VERSION = SPARK_EC2_VERSION
 DEFAULT_SPARK_GITHUB_REPO = "https://github.com/apache/spark"
 
 # Default location to get the spark-ec2 scripts (and ami-list) from
-DEFAULT_SPARK_EC2_GITHUB_REPO = "https://github.com/mesos/spark-ec2"
+DEFAULT_SPARK_EC2_GITHUB_REPO = "https://github.com/amplab/spark-ec2"
 DEFAULT_SPARK_EC2_BRANCH = "branch-1.4"