This updates cloudkittyclient's requirement files in order to use
openstack/requirements for constraints. This will help to avoid dependency
conflicts when cloudkittyclient is deployed in an openstack context.
Work items:
* Updated requirements.txt, test-requirements.txt and doc/requirements.txt
with the `update-requirements` tool provided by openstack/requirements.
* Added a lower-constraints.txt file.
* Added the "check-requirements" zuul job template to the CI.
Change-Id: I12a882ce4d24ade153a64b75852396377ac42ca6
1ed287c92a