[EC2] Factor out Mesos spark-ec2 branch

We reference a specific branch in two places. This patch makes it one place.

Author: Nicholas Chammas <nicholas.chammas@gmail.com>

Closes #3008 from nchammas/mesos-spark-ec2-branch and squashes the following commits:

10a6089 [Nicholas Chammas] factor out mess spark-ec2 branch

ec2/spark_ec2.py

@@ -41,8 +41,9 @@ from boto import ec2
 
 DEFAULT_SPARK_VERSION = "1.1.0"
 
+MESOS_SPARK_EC2_BRANCH = "v4"
 # A URL prefix from which to fetch AMI information
-AMI_PREFIX = "https://raw.github.com/mesos/spark-ec2/v2/ami-list"
+AMI_PREFIX = "https://raw.github.com/mesos/spark-ec2/{b}/ami-list".format(b=MESOS_SPARK_EC2_BRANCH)
 
 
 class UsageError(Exception):
@@ -583,7 +584,13 @@ def setup_cluster(conn, master_nodes, slave_nodes, opts, deploy_ssh_key):
 
     # NOTE: We should clone the repository before running deploy_files to
     # prevent ec2-variables.sh from being overwritten
-    ssh(master, opts, "rm -rf spark-ec2 && git clone https://github.com/mesos/spark-ec2.git -b v4")
+    ssh(
+        host=master,
+        opts=opts,
+        command="rm -rf spark-ec2"
+        + " && "
+        + "git clone https://github.com/mesos/spark-ec2.git -b {b}".format(b=MESOS_SPARK_EC2_BRANCH)
+    )
 
     print "Deploying files to master..."
     deploy_files(conn, "deploy.generic", opts, master_nodes, slave_nodes, modules)