Add "parent" key to list stacks output

Pass back the stack owner_id (parent stack ID) via the RPC API, then
expose it via the "parent" key when listing stacks.  This will be
None for all top-level (non nested) stacks, so can be ignored when
show_nested is not passed.

blueprint: list-nested
Change-Id: I5ad7f867bc7eb138bf2d7d17b4a3bf810d341393
Merge "Add "parent" key to list stacks output"