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. 
This seems like a plugin framework decision. The current working method in a lot of the extended functionality of dokku expects to find app-specific customizations in the root of the app deployment.

Refs:
[dokku-logging-supervisord](https://github.com/expa/dokku-logging-supervisord) 
[zero-downtime deployment](https://github.com/broadly/dokku)

Having said all that, I'm not completely opposed to doing that here. Its just different than other implementations and, in my use case, adds an odd semantic.