Update Dry Run

Performing a stack update is a potentially destructive operation - resources may need to be replaced. The user can get a reasonable idea by scouring the documentation of each property that is changing, but it's a tedious and error-prone process. The user needs to be able to do "heat update --dry-run" or something similar to obtain a list of the changes that will be made for a given update.

(Even better would be if they could constrain the actual update to make only the changes indicated, but this is harder and should be considered out of scope for this blueprint.)