Replace configtxgen usage of viper with explicit decoding into yaml structures

The configtxgen packages use viper and viperutil.EnhancedExactUnmarshal to parse configtx.yaml. With the move away from viper, this should be replaced with vanilla yaml unmarshalling and path reference handling.

As there are a number of places where configtx.yaml uses maps of a user provided name to some structured data (instead of lists of structured data with names), there may be some complications.

It is extremely important that we do not break backward compatibility with the legacy configuration file format but we are removing the ability to override configuration data from environment variables.