Support ServiceClass filtering & manipulation

There are a few cases when deploying a steward in front of a CF broker where the operator may want to filter or manipulate catalog entries from the broker before publishing to the Service Catalog.

- whitelist: pass through services and plans that match a whitelist, deny everything else
- blacklist: drop specific service and plan entries that match, allow everything else
- remap (optional): map service/plan pair from CF to a new service/plan pair in the Service Catalog

Remap is probably the most controversial addition and is most likely optional until someone reeealy needs it.

This issue was moved from https://github.com/deis/steward/issues/229

No files, tests, or entry points are named. Start by tracing how steward-cf obtains broker catalog entries and publishes them to the Service Catalog, then review the moved steward issue 229; define whitelist and blacklist behavior first, and decide whether remapping is in scope. Done means the selected filtering and manipulation semantics are agreed and verifiable.

Written by the indexing model from the issue text.