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.

We still have the other ostestr/testr usage such as tempest run command
and documentations. So, that will be changed with following patches.

Change-Id: Iff4abef50178bdc83b868eed4a906e22d790762b
5 files changed