- 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.