Parameterized HDD, Swap, Memory

In my use of your box, I came to realize that I wanted to easily customize the boxes I was creating (especially disk size).  So, I parameterized HDD size, Swap Size, and Memory size to simplify my life a little bit.  

I also noticed that when I downloaded the Virtualbox guest additions iso, the name used an underscore instead of a hyphen, so I changed that as well.

I also noticed that the virtualbox iso (that I downloaded) had an underscore instead of a hyphen, so I changed that as well.