[SPARK-8596] [EC2] Added port for Rstudio

This would otherwise need to be set manually by R users in AWS.

https://issues.apache.org/jira/browse/SPARK-8596

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

Closes #7068 from koaning/rstudio-port-number and squashes the following commits:

ac8100d [vincent] Update spark_ec2.py
ce6ad88 [Vincent D. Warmerdam] added port number for rstudio