Lazily initialize paths

Currently some path related attributes are constructed in the attributes file, which forces wrapper cookbooks to override `config_dir` even if overriding `install_dir` for example.

This pull request moves all attributes that, by default, are based on other attributes to the `_defaults` recipe, similarly to what's done in the [java](https://github.com/agileorbit-cookbooks/java/blob/master/recipes/set_attributes_from_version.rb) cookbook. I'm not terribly fond of having attributes defined in more than one place, but for now this is the only thing I can come up with that doesn't require a lot of changes.