resource::vhost to set www_root at top level

Added the www_root parameter to a resource::vhost to the user looks like it
would add "root $www_root" to the top level however it just inserts it into
the 'location / { root $www_root; }'. Here we do what the user could expect
resulting in an equivalent configuration for existing users and making it
possible for subsequent resource::location resources not to specify www_root.

This avoids the first pitfall listed here http://wiki.nginx.org/Pitfalls.

This is something that should be done before https://github.com/jfryman/puppet-nginx/pull/199 is merged.