The requirements files have been mostly copied from sahara.git,
but most of the dependencies are not needed.
Refresh the content of lower-constraints.txt and ensure that
the lower-constraints tox environment considers also the doc
Explicitly depend on the pre-release of Sahara for Stein.
Add few more jobs to avoid breakages, namely:
- add check-requirements and openstack-lower-constraints-jobs;
- replace tempest-plugin-jobs with openstack-python-jobs,
openstack-python35-jobs and openstack-python36-jobs
now that it is possible to depend on sahara.