* Add basic docker_compose tests.
* Add more constraints for Py 2.
* Try to install docker-compose from system packages.
* Another try.
* Some more tries.
* One more.
* Move task into block.
* Clean up constraints file.
* More adjustments.
* TEMP: add debug output
* Remove PATH hack
azure-pipelines-test-container:1.7.1 contains a proper fix for ensuring
PATH contains the location of pip installed binaries so we can remove
the hack.
* Correct RHEL groups