[COOK-2395] Add a root path (default to /)

This was originally applied to opscode-cookbooks/application. It turns
out that this is probably only a pattern for php application
repositories. If this functionality is required for other application
types (ruby, python) it can be re-evaluated to add in the base
application cookbook's resource.
[COOK-2395] - adjust app_root attribute

The app_root attribute is moved to this cookbook rather than being in
the application cookbook.