ref(Vagrantfile): align with upstream coreos-vagrant

See "deis" fork at https://github.com/deis/coreos-vagrant
Merge pull request #1881 from mboersma/update-coreos-vagrant

ref(Vagrantfile): align with upstream coreos-vagrant
ref(tests): move test-nightly to test-latest
Merge pull request #1965 from gabrtv/test-latest

ref(tests): move test-nightly to test-latest