Propagate $index_files from vhost to location

The $index_files variable is defined in nginx::resource::vhost, but was not
used in the default nginx::resource::location.  The index_files setting for the
"location /" would therefore always be "index.html index.htm index.php".

This commit propagates this variable from nginx::resource::vhost to
nginx::resource::location, ensuring that the default location also can have a
custom index_files setting, as intended.