Add tests for Cinder user messages v3 API

This commit adds basic tests for the list/show/delete
v3 APIs for Cinder's new user messages.

Depends-On: Id8a4a700c1159be24b15056f401a2ea77804d0a0
Depends-On: I97caa6bfababf7d1cc714296ae66f77d22bf24ab
Depends-On: I80c6a0c46c667291c6f7fe2a036717504c110314

Change-Id: I3d9b3fe288333721bf3b2c6c988949f2f253bfcc
12 files changed