Support DOCKER_OPTS in pillar data

[cdarwin]

As a systems administrator, I would like to adjust docker daemon startup arguments through a known interface.

In Debian this is done through the file, /etc/default/docker. A list of options could be concatenated into an argument for file.append or something similar.

[tiadobatima]

Hi @cdarwin

I submitted #40 to address this issue

Cheers,
g.

[cdarwin]

Thanks, @tiadobatima! 👍