Add Spark to overview and feature matrix

Add Spark to the existing documentation in the overview page and in the
feature matrix for plugins.
Also use the more general 'data intensive application' instead of Hadoop
when referring to the kind of applications Sahara is able to handle.

Change-Id: Ibebeeaada6defa2549026b39e8c6faa23aba9cb8
Merge "Add Spark to overview and feature matrix"