installutils: remove hardcoded subject DN assumption
    
    `installutils.load_external_cert` assumes that the IPA CA subject
    DN is `CN=Certificate Authority, {subject_base}`.  In preparation
    for full customisability of IPA CA subject DN, push this assumption
    out of this function to call sites (which will be updated in a
    subsequent commit).
    
    Part of: https://fedorahosted.org/freeipa/ticket/2614
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>