Add default driver to ldap conf file

From bug http://dev.opennebula.org/issues/2608

source/administration/authentication/ldap.rst

@@ -94,7 +94,7 @@ To enable ``ldap`` authentication the described parameters should be configured.
 
     AUTH_MAD = [
         executable = "one_auth_mad",
-        authn = "ssh,x509,ldap,server_cipher,server_x509"
+        authn = "default,ssh,x509,ldap,server_cipher,server_x509"
     ]
 
 To be able to use this driver for users that are still not in the user database you must set it to the ``default`` driver. To do this go to the auth drivers directory and copy the directory ``ldap`` to ``default``. In system-wide installations you can do this using this command: