Switch to stestr for generating subunit results
Switch to stestr for generating subunit results

* https://review.openstack.org/#/c/504345/ moves the tempest run
  cli to stestr. In order to avoid breakage switch to stestr from testr.
* Check for .testresposity and .stestr folder and then use
  respective test runner. It will avoid the breakage in CI.

Change-Id: I1fdbe2f441a73f80d8d6e7a8263d300939a4fba3

openstack/packstack,openstack/packstack,fr34k8/packstack,stackforge/packstack,mahak/packstack,fr34k8/packstack,stackforge/packstack,mahak/packstack,fr34k8/packstack,stackforge/packstack,openstack/packstack,mahak/packstack