Report missing certificate in external trust chain
    
    When ipa-server-install is called with an external CA, but the cert chain is
    incomplete, the command exits with the following error:
    ERROR CA certificate chain in <list of --external-cert-file> is incomplete
    
    The fix adds in the log the name of the missing certificate:
    ERROR    CA certificate chain in <list of --external-cert-file> is incomplete: missing certificate with subject '<dn of the missing certificate>'
    
    https://fedorahosted.org/freeipa/ticket/5792
    
    Reviewed-By: Jan Cholasta <jcholast@redhat.com>