[SPARK-8596] Add module for rstudio link to spark

shivaram, added module for rstudio install

Author: Vincent D. Warmerdam <vincentwarmerdam@gmail.com>

Closes #7366 from koaning/rstudio-install and squashes the following commits:

e47c2da [Vincent D. Warmerdam] added rstudio module

ec2/spark_ec2.py

@@ -793,7 +793,7 @@ def setup_cluster(conn, master_nodes, slave_nodes, opts, deploy_ssh_key):
             ssh_write(slave_address, opts, ['tar', 'x'], dot_ssh_tar)
 
     modules = ['spark', 'ephemeral-hdfs', 'persistent-hdfs',
-               'mapreduce', 'spark-standalone', 'tachyon']
+               'mapreduce', 'spark-standalone', 'tachyon', 'rstudio']
 
     if opts.hadoop_major_version == "1":
         modules = list(filter(lambda x: x != "mapreduce", modules))