certs: Fix incorrect flag handling in load_cacert
    
    For CA certificates that are not certificates of IPA CA, we incorrectly
    set the trust flags to ",,", regardless what the actual trust_flags
    parameter was passed.
    
    Make the load_cacert method respect trust_flags and make it a required
    argument.
    
    https://fedorahosted.org/freeipa/ticket/4779
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>