Return metadata from resource_signal RPC interface

To enable us to completely remove the metadata_update call without
breaking the current CFN waitcondition API, we need to return the
resource metadata, like metadata_update does (currently we don't
return anything when handling a signal).  This should not be
expensive as the metadata has just been updated, thus should be
cached inside the resource object.

Change-Id: I29bf6f9f5fab133b06ab8ecc92d7556a9bb6e35a
Merge "Return metadata from resource_signal RPC interface"