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. 
I'm 50/50 on whether we should do this. Because rm is destructive, I originally had the user do this manually to understand that this plugin was meant to replace the existing one. There may be rare cases where the user installs this plugin, uninstalls it, and then deployment is broken.

On the other hand, it's kind of obvious that this plugin is meant to replace the existing one. So automating the deletion is convenient.

Perhaps a good compromise is to rename `nginx-vhosts` to `.nginx-vhosts` to hide it from pluginhook. Then make it clear in the README that this is happening.