'Uploads an X.509 signing certificate and associates it with
the specified user.
If the user_name is not specified, it is determined implicitly based
on the AWS Access Key ID used to sign the request.
:type cert_body: string
:param cert_body: The body of the signing certificate.
:type user_name: string
:param user_name: The username of the user'

def upload_signing_cert(self, cert_body, user_name=None):

    params = {'CertificateBody': cert_body}
    if user_name:
        params['UserName'] = user_name
    return self.get_response('UploadSigningCertificate', params, verb='POST')
