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. 
If you're using the dokku_deploy chef [cookbook](http://jonnybgod.ghost.io/using-dokku-with-aws-opsworks/), it creates a VHOST for you. So I figured we could account for the simplest case where we set a default VHOST, while still maintaining per app VHOST functionality.