It turns out that `d.GetOk` also returns `false` when the user _did_ actually supply a value for it in the config, but the value itself needs to be evaluated before it can be used.
So instead of passing a `ResourceData` we now pass a `ResourceConfig`
which makes much more sense for doing the validation anyway.
1. Migrate `chef` provisioner to `schema.Provisioner`:
* `chef.Provisioner` structure was renamed to `ProvisionerS`and now it's decoded from `schema.ResourceData` instead of `terraform.ResourceConfig` using simple copy-paste-based solution;
* Added simple schema without any validation yet.
2. Support `ValidateFunc` validate function : implemented in `file` and `chef` provisioners.
Sander van Harmelen