look for templates in app bundle

This will...

1. Only install nginx if the version is below 1.4.0
2. Removes default nginx-vhosts plugin during install, if it exists
3. Allows override of nginx.*tpl by including it in the app repo/container
4. Looks for VHOST at $DOKKU_ROOT/$APP/VHOST first and then defaults to $DOKKU_ROOT/VHOST

Tested and running on 10 different apps. Definitely open to feedback. 
Can you explain how a root VHOST would be used in this plugin? Because unlike the original `nginx-vhosts` plugin, this one does not automatically create the myapp.vhostname.com hostname based on the root VHOST. 

Thus, as this plugin is now, having a root VHOST would mean that all apps that don't override it will share the same hostname.