When handling the command "openstack stack event list --nested-depth=n", we
obtain the list of nested stacks by querying the database for all resources
that share the root stack ID. However, since all we're getting is the stack
IDs, there's no need to query all of the fields and construct a versioned
object for each resource. Just get the set of stack IDs.
Change-Id: I12155433e2ac1af919aa4b5e780fb965cd5885d8
Related-Bug: #1588561