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.
> but thought maybe there was some voodoo file-locking workaround or something embedded in the two-step approach.

That could've been the case with overwriting the same file via `sed`. :)