Patch to support an LDAP group DN filter

This optional LDAP parameter allows a user to configure an LDAP group DN, which allows for finer granularity when integrating Chef with AD/LDAP.  As it stands today, integrating Chef and AD opens up Chef to every user in the base_dn, which is often a considerably larger universe of users and entities than one wants to access Chef.  This patch must be coupled with the enhancement captured in oc_chef_wm pull request 114 (https://github.com/opscode/oc_chef_wm/pull/114).