Use ember serve host explicitly, instead of inferring

We're using multiple ember-cli apps on the same domain, deployed at different paths. In dev we use nginx to proxy requests to the correct hostname.

Because `window.location.host` is the nginx proxy, and not the ember server, the sweet live-reload breaks.

This proposed change uses the explicit hostname, instead of inferring from window.location, similar to how the `liveReloadPort` value is set explicitly.