Adding ldappool module dependency as needed by keystone bug #1320997.

Ldappool has MPL license and is needed as part of https://review.openstack.org/#/c/95300/
This module provides pooling of LDAP connections.
In keystone, this is optional dependency similar to python-ldap.

Related-Bug: #1320997

Change-Id: Id03a5f42cdc677e9f1f630341a10f6763b010394
Merge "Adding ldappool module dependency as needed by keystone bug #1320997."