Move away from custom data library

[emachnic]

Using the custom data library makes it more difficult for users to override attributes as it locks them in to using a specific data bag. This cookbook should move away from the library and instead just allow users to override the attributes as they wish, either with an attributes file or in a Policyfile or even in their Chef recipe if, for example, they're using Chef Vault for the secrets.

I'd be willing to take on this task as well as Windows support but I wanted to see the thoughts before submitting a Pull Request for that work.