Option to create Ubuntu 14.04 clusters using spark-ec2

[manubansal]

Here's a beta version of Ubuntu 14.04 addition to spark-ec2. There are some simplifications made for the Ubuntu create/setup sequence where Rstudio, ganglia, and persistent-hdfs have not been ported to include Ubuntu. All necessary setup is functional and users can run spark jobs on Ubuntu clusters. No functional changes have been made to the default spark AMI.

This PR should be treated as a trial before declaring stable. We have been running tests to create, start, stop, destroy Ubuntu clusters, and using those clusters for running spark jobs. However, tests are not yet exhaustive across different instance types or input options, so the coverage is limited. We consider this version usable at large with the understanding that minor bugs could show up.

[akouri-tesla]

Thanks for doing this! I could really use this!