Create socket_group option, which gets passed in as the -G option.

https://github.com/dotcloud/docker/pull/3612 added a `-G` option to docker that allows the user to control what group owns `/var/run/docker.sock`. This patch makes it possible to specify this parameter using `socket_group => `

I'll have another similar pull request soon, so I'm considering refactoring the way parameters get constructed - it might make sense to pull the logic out into something that can be shared by both the RedHat and Debian templates.