Added CustomerUserMemberAttr option to enable support for Microsoft AD

The LDAP CustomerUser backend does a check on line 419 to filter out users who are not a member of a configured group in LDAP. The hardcoded attribute 'memberUid' is used to perform the check. In a Microsoft Active Directory server this attribute should be named 'member' and it's corresponding value is the Distinguished Name (DN), not the user id.

I added the option "CustomerUserMemberAttr". Microsoft AD users should set this option to "member". If you leave it empty it defaults to "memberUid" and the behaviour of the LDAP CustomerUser backend is not changed.