Move API parameter parsing from OpenStack API to engine

In commit 4169c1bd8ce4db0a794e22471994ea401b77b9c0, the API parameter
parsing was moved to the OpenStack API.  Since then, inputs to the CFN
API were not being validated, creating a security hole.

Change-Id: I21920591075bcefbe695316dab6605afd6f4ec64
Closes-Bug: #1317667