Fix stack update for Python API

Change I30a2b223d7dd19a717ed989b419b1e6cb2ac143b
introduced 'existing' keyword to stack-udpate command that became effectively
required argument to stack.update method of the client's Python API,
breaking backward compatibility.

Change-Id: Icda3313805c29ab36773a4edc8c9f8c874b71ca3
Closes-Bug: #1371167
Merge "Fix stack update for Python API"