Expose resource dependency required_by to REST API.

Each resource can generate a list of names of resources which
require this resource as a direct (non transitive) dependency.

This information is returned in the list as well as the show REST calls
so that a diagram of the running stack can be built with a single
request.

Other uses of exposing this information is:
- template authors debugging their own template dependency issues
- integration tests validating template dependencies

Change-Id: Ibe62345afa87e49c4e2152a5fcb74e5ee003124e
Merge "Expose resource dependency required_by to REST API."