Refactor installer to simply wrap the Docker Toolbox installer

[embray]

I started out here by simply forking Docker Toolbox and modifying its installer to also include and install the image for sagemath (or sagemath-jupyter, specifically).

This was the easiest way to get something off the ground, but in retrospect I think it needs to be reworked--we need our own installer (though we can still reuse a lot of the Docker Toolbox one), and we need to simply include the Docker Toolbox installer in our installer, and install it as a separate product.

The current approach can actually end up providing the user with two separate copies of Docker on their system if they run both the Sage installer, and the Docker installer separately (likely at different workshops, for different purposes, months apart from each other, with no idea what they did previously).

So yes, a safer, and in fact simpler approach will be to include the Docker Toolbox installer separately, and run it as part of our install. This can include a check whether Docker is already installed, as well as whether it can be upgraded, and give the user the option to upgrade their Docker install if the one we include is newer. If necessary we could also require a minimum version of docker. I don't know right now what the minimum version should be, but we can leave that out for now until experience demonstrates a need for it.

Note, there are some slight problems with customizing automated installation of Docker Toolbox. For example it's currently impossible to deselect Git (even though I don't believe it to be necessary for the installation). I've tried to address this in docker-archive#418. In the meantime it will install Git, but that should be relatively harmless, I guess...