This commit switches the ostestr command to use stestr under the covers.
This simplifies the majority of the logic, eventually ostestr will
be deprecated as a result of this migration. (since almost all the logic
here is contained in stestr already)
Depends-On: I2c7618a742439fd2ed26879f3114f0f66fd6337f
Change-Id: Id7cb2a39a8308f1413608dcf19273a1d7f33592e