Logging settings for peer are not flexible enough

Currently, the log level for the peer can be defined in core.yaml logging section with some additional fine grained log settings for some fixed loggers such as gossip, ledger, grpc, ... .

Those loggers are hardcoded in the peer node here:

https://github.com/hyperledger/fabric/blob/release/peer/node/start.go#L246

However, in case we want to enable debug logging for some system chaincode, e.g. vscc, currently one has to set the peer log level to debug. This results super verbose logging of the peer. 

It should be possible to add any logger to the logging section of the core.yaml in order to override the peer logging settings.