Eager fetch stack tags with a subqueryload
Eager fetch stack tags with a subqueryload

Currently doing a stack-list results in one SQL select for the stacks
and then one SQL select per stack for its tags.

By fetching the tags with a subqueryload, stack-list calls will only
do 2 selects, one for the stacks, and one for all of those stack's
tags.

Partial-Bug: #1578851
Change-Id: If8b3cd9d034859b71621afe7db15b9a32bc8aea6

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