Allow Fuel to be integrated in corporate DNS

One of Mirantis customers has a use case where they want to use the same domain for Fuel as they do for the rest of their network. This creates a problem even on deployment stage if they, for example, specify NTP servers as hostnames from their domain, the same domain entered during Fuel installation. Deployment will fail because Fuel will not be able to resolve these hostnames. We might want to look into options which would allow such scenario to work, or at least provide recommendations in our documentation related to DNS integration in a domain environment.