adds aws_ebs_volume recipe for creating EBS volume on EC2 instance

We use Amazon EC2 as infrastructure for our mongo setup. 
Amazon has a really good [guide](http://media.amazonwebservices.com/AWS_NoSQL_MongoDB.pdf) for setting up mongos on EC2.
They strongly recommend to use separate EBS volume for DB data storage.
I created a recipe according to this guide, hope it will be useful not only for me :)