Genericizes the yaml+environment versioned configuration parser

Registry configuration parsing uses the new parser with a single version
declaration and an environment prefix of "REGISTRY"
Allows storagedriver parameter values to be of type interface{}

This enables use of nil, booleans, numeric types, and even complex structures for parameter values, assuming they can be parsed from yaml.
Merge pull request #861 from BrianBland/ng-configuration

Genericizes the yaml+environment versioned configuration parser