Move advertising services logic to a separate class.
Rename collection getter

[#48753337]
Prepare gateway to use unique id to update offerings.

 * Extract Service, Plan and ServicePlanChangeSet into model objects
 * Extract HTTP handling into its own object
 * Extract CCNG advertising into ServiceAdvertiser
 * Rename CloudControllerCollectionGetter to CloudControllerServices

[#48753337]
Rename the test for AsynchronousServiceGateway
Fix a bug where plan name was not set properly

Also Extract logic for generating GW catalog
[#48753337]
Gateway updates services and plans based on unique_id

Don't ignore services without registered auth tokens. We'll ignore
unknown unique_ids anyway.

[#48753337]