[EC2] don't duplicate default values

This PR makes two minor changes to the `spark-ec2` script:

1. The script's input parameter default values are duplicated into the help text. This is unnecessary. This PR replaces the duplicated info with the appropriate `optparse`  placeholder.
2. The default Spark version currently needs to be updated by hand during each release, which is known to be a faulty process. This PR places that default value in an easy-to-spot place.

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

Closes #2290 from nchammas/spark-ec2-default-version and squashes the following commits:

0c6d3bb [Nicholas Chammas] don't duplicate default values