Part of Support Nested Stacks and Updates story
To add a minimally viable amount of Heat stack-update support to Valet,
significantly restrict the number of update use cases using a set of acceptance
criteria. Skip holistic placement at stack-update time in favor of Valet's
existing re-plan mechanism, placing or replacing resources one at a time, albeit
still in consideration of other resources in the same stack hierarchy.
Change-Id: Id44819f1b69dc2da2b026bdbf3d9c93fed9adc11
Story: #2001139
Task: #4856
68c9b65598