Revert "Add CNAME option"

Reverts progrium/dokku#657

There appeared to be a few issues with this merge, and there is a plugin that handles domain configuration quite nicely. Handling CNAMEs in a way that doesn't follow other platforms - heroku being the main one - means that it is less likely that we can manipulate this option via a third-party tool - ruling out any dashboard-like software - and thus I think the merge was ill-advised.

A proper implementation would not depend upon a file in the repository, as that would tie that repository to a single host, and thus not allow it to be deployed by multiple users.