Decouple Nested Stacks from parents

Initial draft of idea for interim scalability improvement for
scenarios where a large heirarchy is of nested stacks is deployed,
if we can decouple the nested stacks from their parents, then we
should be able to more effectively spread workload over multiple
engines or workers, whereas currently the entire tree of stacks
must be processed by one engine.

specs bp decouple-nested

Change-Id: Ic6e955733030c80982221338a6d99c9c67cf064c
Merge "Decouple Nested Stacks from parents"