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.
@mboersma wouldn't it actually make more sense to run `make discovery-url` here? I don't really see a use-case when you would want a not provisioned vagrant instance joining an existing cluster and this could be still made possible with an ENV var.