Change collector interface to return a list

With the new OS::Heat::StructuredDeployment resource, each Metadata
section may have multiple "deployments" in it. With this, we will return
a list with tuples of key and content to write to the cache.

Change-Id: I9f4272b0761e1dfd850bc5a5c6b27a78f126281f
Related-Bug: #1295787