Update ubuntu packaging script, especially to stop docker group deletion

This was based both on the fact that the group deletion issue described in https://github.com/dotcloud/docker/pull/2226#issuecomment-26607675 wasn't actually fixed, and then on some subsequent discussion with @paultag to make sure our deb packages are more correct than they've been (we can't be 100% correct without using something official like "debhelper" and proper packaging, but this is definitely a good move towards being proper).

So the proper fix for our group deletion is to just never delete the group, which makes for a _much_ better user experience. :)