Introduce Variant of Stack Update That Takes Deltas Instead of Whole Revised Template

Introduce into the Heat API a variant of the stack update operation that takes, instead of a whole new template, some deltas to apply to the stack's template. The semantics are defined to be to: lock the stack, get the stack's current template, apply the deltas, do a regular stack update with the revised template and the stack's current environment, and unlock the stack.