Add docker group if missing
Add docker group if missing

Vagrant upstream expects the docker group. As a result, add it if it's
missing. It turns out it's not just atomic which needs this, as newer
fedora/centos releases don't include it by default.

This patch adds it into the "run once" section. As a result, if docker
is not set to true on the first boot, it won't be included
automatically. This is optimized to save unnecessary wait time.

bhalothia/oh-my-vagrant,mairin/oh-my-vagrant,mairin/oh-my-vagrant,geekcheng/oh-my-vagrant,goern/oh-my-vagrant,gpjonesii/oh-my-vagrant,purpleidea/oh-my-vagrant,gpjonesii/oh-my-vagrant,br0ziliy/oh-my-vagrant,goern/oh-my-vagrant,LalatenduMohanty/oh-my-vagrant,purpleidea/oh-my-vagrant,geekcheng/oh-my-vagrant,LalatenduMohanty/oh-my-vagrant,br0ziliy/oh-my-vagrant,LalatenduMohanty/oh-my-vagrant,mairin/oh-my-vagrant,gpjonesii/oh-my-vagrant,bhalothia/oh-my-vagrant,ncoghlan/oh-my-vagrant,ncoghlan/oh-my-vagrant,br0ziliy/oh-my-vagrant,goern/oh-my-vagrant,geekcheng/oh-my-vagrant,bhalothia/oh-my-vagrant,purpleidea/oh-my-vagrant