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. Derived from work done by @nathansamson.

Closes #1784, refs #2098.
`user-data.new` is just a tempfile used by `make discovery-url`--see the next line. I'm not entirely sure why it was done this way, so I was respectful of convention.