Do a single query to fetch all event_list stacks
Do a single query to fetch all event_list stacks

In preperation for supporting event list --nested-depth, do a single
query to fetch the stacks that correspond to the event list.

Related-Bug: #1588561

Change-Id: I106c21bbb87b9a591f3aba7de03eb130e7fd7b19

noironetworks/heat,noironetworks/heat,cwolferh/heat-scratch,openstack/heat,openstack/heat,cwolferh/heat-scratch