[COOK-4192] Add attribute to control multisite features

WordPress includes multisite features for running a network of sites. 
(See http://codex.wordpress.org/Create_A_Network for details.) These 
must be enabled by editing wp-config.php, which is managed by chef when 
using the wordpress cookbook.

This patch adds a boolean attribute, node['wordpress']['allow_multisite'], 
to enable the multisite features.

Fixes: https://tickets.opscode.com/browse/COOK-4192