Make python-ldap optional for PyPI packages
    
    python-ldap is a Python package with heavy C extensions. In order to
    build python-ldap, not only OpenLDAP development headers are necessary,
    but also OpenSSL, Cyrus SASL, and MIT KRB5 development headers.
    
    A fully functional ipaclient doesn't need an LDAP driver. It talks JSON
    RPC over HTTPS to a server. python-ldap is only used by ipapython.dn.DN
    to convert a string to a DN with ldap_str2dn(). The function is simple
    and can be wrapped with ctypes in a bunch of lines.
    
    Related: https://pagure.io/freeipa/issue/6468
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Alexander Bokovoy <abokovoy@redhat.com>