Add stack-update argument for patching parameters

A new argument named existing for stack-update will be
supported by the heat engine to use the parameters stored
in the Db.

Partially-implements: blueprint troubleshooting-low-level-control
Partial-Bug: 1224828
Change-Id: I30a2b223d7dd19a717ed989b419b1e6cb2ac143b
Merge "Add stack-update argument for patching parameters"