[keystone] Switch to stestr
[keystone] Switch to stestr

Keystone already switched to stestr in [1], change
the spec as well to use stestr.

[1] https://review.openstack.org/#/c/508611/

Change-Id: I7ed59937b8fe583447fb158f90088e3f4a4dba7c

openstack/rpm-packaging,openstack/rpm-packaging