Optimize the service catalog lookup construction process

[arschles]

In #72, the service catalog is constructed by:

1. Fetching all available services from the backend CF service broker
2. Trying to publish all those services to the k8s ServiceCatalogEntry third party resource (ignoring errors that the entry already exists)
3. Fetching the catalog from the k8s ServiceCatalogEntry third party resource

Steps (2) and (3) can be merged as an optimization. Already-existing entries and newly published ones should be written to the lookup table at the same time, and we'll only have to write ServiceCatalogEntrys once.

[arschles]

#87 will be useful in unit testing this change

[arschles]

We've split steward into steward-framework, the Go SDK for Kubernetes resources and control loops, and steward-cf, the CF broker API implementation on top of steward-framework.

steward-framework constructs the service catalog differently (according to ServiceClass described in kubernetes-retired/service-catalog#31), so this issue no longer applies; closing.