Fixes #198 (inconsistent fieldnames in model/broker RESTful APIs)

This pull request resolves the issues we uncovered recently with inconsistent fieldnames in the model/broker RESTful APIs. In short, the following changes were made in the RESTful APIs for those slices:

* the input `req_metadata_hash` parameter in the `POST` and `PUT` operations in these two APIs was renamed; with this PR in place, that parameter is renamed as the `req_metadata_params` (since it contains the parameter values for a given model/broker); this should avoid confusion of this parameter with the `req_metadata_hash` instance variable in the underlying model/broker object.
* the underlying model and broker base classes where refactored to add some additional utility methods that were needed to properly handle the inputs from the user through the CLI and to properly handle the inputs to the RESTful API.
* the associated slice classes were refactored to properly construct the RESTful API calls associated with the new RESTful API arguments.
* additional error handling was added within the RESTful API to deal with errors that might be present if users directly access the RESTful APIs for these slices (instead of using the CLI)

Overall, this should provide developers with a much clearer codebase, making continued support of this codebase much simpler over time (and making the API much clearer than it was previously).