Prevent searching for AD groups for non AD LDAP

Searching non AD LDAP domains for AD groups will result in many entries
in the error.log. A new setting has been added to make it easier to
differentiate between AD and posix LDAP. Default is AD to make sure
currently functionality is not changed and ensure save upgrade.

Example usage:
$wgLDAPActiveDirectory = array(
    'testADLDAPDomain' => true, 'testPosixLDAPDomain' => false
);

Change-Id: Id7eb016a3c105fa9e08592f876bd5a8726416b68