spec file: clean up BuildRequires
    
    Add missing cyrus-sasl-devel, python-cffi, python-custodia,
    python-dateutil, python-nose, python-paste, python-sss-murmur,
    python-sssdconfig and systemd-python BuildRequires.
    
    Remove unused custodia, java-headless, m4, policycoreutils,
    python-kdcproxy, python-rhsm, pyOpenSSL and systemd-units BuildRequires.
    
    Correct versioned BuildRequires and provide explanatory comments.
    
    https://fedorahosted.org/freeipa/ticket/6418
    
    Reviewed-By: Petr Spacek <pspacek@redhat.com>
    Reviewed-By: Martin Basti <mbasti@redhat.com>