Since ostestr switched to running stestr under the covers, we lost the
old magic setting of the environment variables via .testr.conf for
capturing stderr/stdout and the test timeout. This makes the unit,
functional, and api-samples envs consistent with the py27 env that was
already updated to set those variables.
This also updates the pretty_tox3.sh script to run stestr directly.
Change-Id: I27fa9b7e25c1a1dc921653eec84864423f898a85
53f244da12