Merge pull request #12 from shivaram/v2

Set hadoop home to /root/mapreduce for MR1
Using CloudFront URL's
Spark version bump
Merge pull request #13 from pwendell/v2

Use CloudFront URL's instead of direct ones.
Removing now un-needed mounting steps.

1. The ephemeral drives are now mounted according to the /etc/cloud.cloud.cfg
   script on the instances.
2. We no longer need /cgroups.
Adding defensive exists to init scripts
Merge pull request #14 from pwendell/v2

Two minor fixes
New AMI's
Add github.com to known_hosts