* Adding yaml linter to ci check
* Minor linting fixes from yamllint
* Changing CI to install python pkgs from requirements.txt
- adding in a secondary requirements.txt for tests
- moving yamllint to tests requirements
- Only have ubuntu to test on
- fedora and redhat are placeholders/guesses
- the "old" package repositories seem to have the "new" CE version which is `1.13.1` based
- `docker-ce` looks like it is named as a backported `docker-engine` package in some
places
- Did not change the `defaults` version anywhere, so should work as before
- Did not point to new package repositories, as existing ones have the new packages.
This will allow to use '-e docker_version=1.12' in ansible playbook
execution. It's also backward-compatible and will work with floating
docker_version format in custom yaml files.
Closes#702
