Support provider-specific params

[aweiteka]

This is just a placeholder to consider yet another parameter scope: those params that are only related to a specific provider. We expect some providers to have many more parameters than others. For example, typically the docker provider will have fewer params than the kubernetes provider.

[aweiteka]

This may be partially, if not fully, addressed by #70 since referencing params by provider-specific path is inherently provider-specific.