Tutorial environment upgrade notes -  699

Here are a couple of options for upgrading the tutorial environment. 
 1. Destroy and recreate
 2. yum upgrade to the desired flocker-node version

I wasn't sure whether to include the second option, but it might be interesting to people who are familiar with yum and RPMs. Also might be better for those people who have gone beyond the tutorials and created their own application configurations within our tutorial environment. Perhaps I ought to add those reasons to the document.

The yum upgrade option works because #510 has now been resolved. So in fact the `yum install` lines can be simplified, which I'll do in a separate pull request (#702).

Fixes #699
Maybe make `VirtualBox` a link so people can find relevant docs slightly more easily.