Replace *all* references to nat network with Default

These are only descriptions but we might as well be consistent. The connection
description is a bit harder because there are a few 'rasd:Description' field.
Update README with tip about vagrant destroy
Add instructions for testing against Skyscape

We haven't previously included this because it's too tightly coupled to a
particular vendor, but since that's what we're using and it's not an easily
memorable command...
Add backslashes to denote command runs over lines
Merge pull request #3 from alphagov/default-network-and-testing

Default network and testing