Update python-ldap version for Keystone

We are currently pinned to an old version of python-ldap. Version
2.4 of python-ldap has been out for over 3 years now, and most
platforms are shipping it.  We should be testing against it instead
of using an old version.

Change-Id: Ifbbcfe2ccf2caa70758f75e44d526d23d7f466cb
Merge "Update python-ldap version for Keystone"