Retreives the CA associated with a group.Show details from next link. https://docs.aws.amazon.com/ja_jp/cli/latest/reference/greengrass/get-group-certificate-authority.html
| aws greengrass get-group-certificate-authority \ |
| Group Certificate Authority Properties - AWS Greengrass : http://docs.aws.amazon.com/greengrass/latest/apireference/-greengrass-groups-groupid-certificateauthorities-certificateauthorityid.html |