Allow users to pass a checksum for the Enterprise package

The `checksum` attribute of the `remote_file` resource prevents the package from being re-downloaded if it already exists in the Chef cache. This pull request allows users to provide a checksum namespaced under `platform` (`debian`, `ubuntu`, `centos`) and `platform_version` (`7`, `precise`, `6`).

For example:

```ruby
# Riak Enterprise 1.4.0 on Ubuntu 12.04
node['riak_cs']['package']['checksum']['ubuntu']['precise'] = "3f29736ab0dc8b094272d9caefed4e795ae293f54f79bed8d0464ca821c865ef"
# Riak Enterprise 1.4.0 on CentOS 6.4
node['riak_cs']['package']['checksum']['centos']['6'] = "a2234d09bdbf08da8b9bf5627b4ea718772fc4964c4d59bd3d4c8ada83d5a9f1"
```