Scenarios for promoting artifacts across registries

[reasonerjt]

By reading the scenarios doc:
https://github.com/notaryproject/requirements/blob/master/scenarios.md

The step 2 -> step 3 in the End to End Orchestrator Scenario seems not covered in the senarios breakdown.

Another scenario such as promoting artifacts across registries should be added, in addition to Scenario #4: Promote Artifacts Within a Registry, Using a Different Repo