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.

Signed-off-by: Harrison Conlin <me@harrisony.com>