Switch to use stestr for unit tests directly

This commit switches to use stestr command to run unit tests directly.
Recently, the latest ostestr command uses stestr instead of testr.
However, we should use stestr directly because it's simple and
straight forward. There is no reason to use ostestr for unit tests
anymore.

Modeled after Iff4abef50178bdc83b868eed4a906e22d790762b

Change-Id: Ie50cd57e87bc049c29b86fce67cc1a2061dad690
4 files changed