Make custom Vagrant settings less shouty

An early patchset of I689f7c129 used a bash script to declare these variables,
which informed the decision to name them using all-capital letters. But it's
a bit shouty and obnoxious, so switch to lowercase letters instead.

Change-Id: I40942283aa2a774e4d8a9fde280b2700719c633d
Merge "Make custom Vagrant settings less shouty"