According to Openstack summit session ,
stestr is maintained project to which all Openstack projects should migrate.
So we should switch to stestr.
Since pbr already landed and the old version of hacking seems not
work very well with pbr>=2, we should update it to match global
Add structure, unit tests, and template for mistral-specs, creating
a functional specs repository without any approved specs.
Put example.rst in mitaka/approved and mitaka/implemented respectively
to avoid doc build warnings:
"pattern u'specs/mitaka/approved/*' didn't match any documents"
Implements: blueprint mistral-spec-repository