Pass flag to engine service to patch parameters

For stack-update, add a new PATCH ReST API and a new CLI
argument named existing-parameters to indicate that the set
of parameters should be patched over the existing parameter
from the DB.  A new method in environment handles
the patching.

Partially-implements: blueprint troubleshooting-low-level-control
Partial-Bug: 1224828
Change-Id: I802e0dca44926be3a3f45fcaa995c866a4abf998
Merge "Pass flag to engine service to patch parameters"