Address the Evan TODOs in the Nozzle

[mattysweeps]

https://github.com/cloudfoundry-community/stackdriver-tools/blob/master/src/stackdriver-nozzle/config/config.go

contains a few TODOs:

• Determine which flags should be required (in Config)
• Validate configs for both Firehose and RLP modes