Add clean make target

Author: timofurrer

GNUmakefile

@@ -108,3 +108,6 @@ TERRAFORM_VERSION = v1.1.4
 tool-terraform:
 	@# See https://github.com/hashicorp/terraform/issues/30356
 	@[ -f $(GOBIN)/terraform ] || { mkdir -p tmp; cd tmp; rm -rf terraform; git clone --branch $(TERRAFORM_VERSION) --depth 1 https://github.com/hashicorp/terraform.git; cd terraform; GOBIN=$(GOBIN) go install; cd ..; rm -rf terraform; }
+
+clean: testacc-down
+	@rm -rf certs/