Unit tests for RunnerParams.

Small changes to make this easier - mostly just defining constants.
Generalize RunnerParams.

Create ParamsWithSpecs class in galaxy.util. If we are going to build out specs for all LWR client parameters - would be nice if they could be reused on the LWR server side where relevant (e.g. AMQP connection parameters).

Slightly modified implementation that extends collections.defaultdict so that operations like iterating over parameters, fetching keys, etc... are available so LWR runner param handling code can look like destination param handling code.