LDAP: Support custom memberOf attribute

The current LDAP authentication module in Observium does support custom names for a specific set of attributes (uid, uidNumber, cn, dn), but is lacking support for providing a custom attribute name for memberOf.

Our company is using a customized LDAP scheme which maps the users roles into one or more AppRoleDN attributes. This makes it currently impossible to properly use Observium, as the built-in user list always appears empty due to the inexistant memberOf attribute(s).

This patch adds a new configuration option $config['auth_ldap_attr']['memberOf'] which allows Observium to look for a different attribute name instead of the hardcoded memberOf attribute. It has been successfully tested in our environment against both stable and trunk.