This commit is a follow-up of "switch to using stestr"
which was merged already [1].
After switch to using stestr, testrepository and .testr.conf
are unnecessary and should be removed.
This patch also switch from "test" to "stestr" for coverage job .
[1] https://review.openstack.org/571716
Change-Id: I6b506a19edf06a5bc40472f84a51eaae98583a62