Support for r3 instance types.

Amazon's new r3 instances come without EXT3-formatted drives. This patch
adds a workaround that detects this situation and formats the drives
as ext3.

Hopefully this will be subsumed later if anyone wants to pick up
SPARK-1079.
Using XFS instead of ext3
Small fix
Merge pull request #53 from pwendell/v3

Support for r3 instance types.