[SPARK-787] Add S3 configuration parameters to the EC2 deploy scripts

When deploying to AWS, there is additional configuration that is required to read S3 files. EMR creates it automatically, there is no reason that the Spark EC2 script shouldn't.

This PR requires a corresponding PR to the mesos/spark-ec2 to be merged, as it gets cloned in the process of setting up machines: https://github.com/mesos/spark-ec2/pull/58

Author: Dan Osipov <daniil.osipov@shazam.com>

Closes #1120 from danosipov/s3_credentials and squashes the following commits:

758da8b [Dan Osipov] Modify documentation to include the new parameter
71fab14 [Dan Osipov] Use a parameter --copy-aws-credentials to enable S3 credential deployment
7e0da26 [Dan Osipov] Get AWS credentials out of boto connection instance
39bdf30 [Dan Osipov] Add S3 configuration parameters to the EC2 deploy scripts
Add a Community Projects page

This adds a new page to the docs listing community projects -- those created outside of Apache Spark that are of interest to the community of Spark users.   Anybody can add to it just by submitting a PR.

There was a discussion thread about alternatives:
* Creating a Github organization for Spark projects -  we could not find any sponsors for this, and it would be difficult to organize since many folks just create repos in their company organization or personal accounts
* Apache has some place for storing community projects, but it was deemed difficult to work with, and again would be some permissions issues -- not everyone could update it.

Author: Evan Chan <velvia@gmail.com>

Closes #2219 from velvia/community-projects-page and squashes the following commits:

7316822 [Evan Chan] Point to Spark wiki: supplemental projects page
613b021 [Evan Chan] Add a few more projects
a85eaaf [Evan Chan] Add a Community Projects page