COOK-4318 - use node.platform for amazon packages

https://tickets.opscode.com/browse/COOK-4318


The package list built for amazon platform is based on `platform_version`.
And it supports only version `2013.xx`, not `2012.xx`.

See https://github.com/opscode-cookbooks/mysql/blob/a68479e2ded438b0a441f3ffbd1de4c71395cb8d/attributes/server_rhel.rb#L18

Instead of using `platform_version` attribute, the `platform` attribute should be used.