Add CloudFormation commands to retrieve stack resouces

@timkay not sure if you had plans to add CloudFormation support. I like the simple deployment of this aws script, but found that I needed the DescribeStacks and DescribeStackResource command to pull the CF metadata when bootstrapping an instance. I added just the basic commands in a straightforward way. If there is a better way to accomplish this within this package, I'm happy to fix it.