Convert /v3/users to use flask native dispatching.
The following test changes were required:
* Application Credentials did not have the plural form
in the JSON Home document. The JSON Home document was
corrected both in code and in tests.
* Application Credentials "patch" test needed to be
refactored to look for METHOD_NOT_ALLOWED instead
of NOT FOUND for invalid/unimplemented methods.
The "assertValidErrorResponse" method was
insufficient and the test now uses the flask
test_client mechanism instead.
Change-Id: Iedaf405d11450b11e2d1fcdfae45ccb8eeb6f255
Partial-Bug: #1776504