Attempt to use a new Mash object that auto-vivifies (auto-creates)

during the gitlab.rb parsing.

It's called vivify because that's what Chef calls it in their VividMash, and that's where we are using the [] and convert_value methods from to create ours.

It really just means it auto-creates a new Mash when you ask for a key that doesn't yet exist.

We don't want this on all the time, because we often check these keys as booleans to determine if they are set (and expect them to return nil)

So we only auto-vivify during the from_file call.

cc\ @ibaum