add --use-iam flag to backup

subcommands/backup

@@ -7,12 +7,12 @@ source "$(dirname "$(cd "$(dirname "${BASH_SOURCE[0]}")" && pwd)")/functions"
 postgres-backup-cmd() {
   declare desc="creates a backup of the $PLUGIN_SERVICE service to an existing s3 bucket"
   local cmd="$PLUGIN_COMMAND_PREFIX:backup" argv=("$@"); [[ ${argv[0]} == "$cmd" ]] && shift 1
-  declare SERVICE="$1" BUCKET_NAME="$2"
+  declare SERVICE="$1" BUCKET_NAME="$2" S3_FLAG="$3"
 
   [[ -z "$SERVICE" ]] && dokku_log_fail "Please specify a name for the service"
   [[ -z "$BUCKET_NAME" ]] && dokku_log_fail "Please specify an aws bucket for the backup"
   verify_service_name "$SERVICE"
-  service_backup "$SERVICE" "$BUCKET_NAME"
+  service_backup "$SERVICE" "$BUCKET_NAME" "$S3_FLAG"
 }
 
 postgres-backup-cmd "$@"