fix(coreos-install): Fix compatibility with older versions.

When adding proper support for selecting release channels I forgot to
add the special case to make `-V alpha` continue to work.

Fixes https://github.com/coreos/bugs/issues/29
Merge pull request #104 from marineam/compat

fix(coreos-install): Fix compatibility with older versions.
feature(network-online.target): ensure IP routable before enabling

This adds a new service to delay network-online.target until the box has a routable IP.