Added ability to deploy to multiple custom domains in VHOST.

- nginx-vhost plugin now no longer manages `$DOKKU_ROOT/$APP/VHOST`
  file but rather reads from it.
- nginx-vhost plugin supports multiple custom domains in VHOST.
- Default nginx configuration template pulled out into separate
  files instead of adding as a HEREDOC in `post-deploy`.
- Added helper messages during deploy.
- Removed nginx-vhosts commands since not necessary anymore.
- Removed root VHOST setting under install in favor of a separate
  domains plugin managing VHOST settings.
- Minor code cleanup.
- NOTE: Setting the `URL` for multiple VHOSTs needs refactoring.