* Add openssh-client to default docker container.
* Include Azure requirements in default container.
To do so, handling of pip requirements was updated to install each
set of requirements separately and then run a verification pass to
make sure there are no conflicts between requirements.
* Add missing --docker-no-pull option.
* Add documentation for the azure-requirements test.
* Handle old versions of coverage.
* Handle old versions of setuptools.
* Detect python version for docker/remote units.
* Add sanity override for test constraints.