Lower python-ldap requirement for F27
    
    For DNSSEC daemons on Python 3, python-ldap requirement was bumped to
    python-ldap 3.0. But python-ldap 3.0 hasn't been released yet and is
    only available as beta4 on rawhide. The DNSSEC fix hasn't landed either.
    
    Lower requirements to python2-ldap 2.4.15 and python3-pyldap 2.4.35.1-2
    until the DNSSEC fix has landed.
    
    See https://pagure.io/freeipa/issue/7257
    
    Signed-off-by: Christian Heimes <cheimes@redhat.com>
    Reviewed-By: Rob Crittenden <rcritten@redhat.com>