* feature: install project atomic CSS on RedHat family

* missing patch for this feature

* sub-role refactor

* Yamllint fix

* 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

* Make yum repos used for installing docker rpms configurable
* TasksMax is only supported in systemd version >= 226
* Change to systemd file should restart docker

- Remove variable dup for Ubuntu
- Update Docker to 1.13

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

On CoreOS where there is no package management, perform zero-trip
loops instead of throwing an exception for iterating over a member
of an undefined variable.