ref(user-data): "make discovery-url" copies from user-data.example

This also adds contrib/coreos/user-data to .gitignore, so that local
changes to this essential file do not cause "git status" to mark the
repo as dirty.
fix(Vagrantfile): require user-data when provisioning
Merge pull request #2118 from mboersma/fix-user-data

ref(user-data): "make discovery-url" copies from user-data.example