Support NO_VHOST to prevent nginx vhost

This change allows you to use "dokku config:set [app] NO_VHOST=true" which will then prevent creation (and deletion if it already exists) of the nginx.conf and URL files.

Makes it more practical to deploy internal applications (ex: a Java back-end which is called by a Node front end), which are better linked using named container links (using the dokku-name and dokku-link plugins)