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 like the idea of having overrides in the app's repo. However, I think the file organization could be better. What do you think about having these overrides in a folder called `.dokku/` that is checked into the app's repo? That way, these files can be kept reasonably separate from app code.

On the other hand, if this is a good idea, perhaps it can be spun off into its own plugin. Like: dokku-config-files where the plugin simply copies all files from `.dokku/` to the `$APP_PATH/`.