See: http://tools.ietf.org/html/draft-ietf-sidr-rescerts-provisioning-09#section-3.4.2
This type of class element contains the signed certificate for this request/response
as oposed to all current certificate elements as used in the list response.
See RFC8183 section 5.2.4:
"The <repository_response/> message is a repository's response to a
publisher which has previously sent a <publisher_request/> message."
See: http://tools.ietf.org/html/draft-ietf-sidr-rescerts-provisioning-09#section-3.3.2
This type of class element contains a current certificate element for each key pair,
as opposed to just one in the CertificateIssuanceResponse.