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.
Ability to override default nginx .conf files with user-defined ones.

If `post-deploy` detects that either `nginx.ssl.template`,
`nginx.once.template`, or `nginx.template` exist in `$DOKKU_ROOT/$APP/`,
then the `post-deploy` script will use those over the default ones.