When developing Dokku, it is useful to use a real domain to test actual functionality. This is impossible if the domain is hardcoded to dokku.me in testing.
Previously, the shorthand was not properly supported. While this _should_ be the domain of the gliderlabs/herokuish repository, we do it here as a courtesy to Dokku users.
Additionally, support the heroku-community => heroku mapping.
Closes#4452
