terraform: Fix cleanup of tfstate backups

Terraform leaves timestamp-based tfstate backups in the terraform
directory, which don't need to remain after a destroy. This patch fixes
the problem by adding the remaining files to the cleanup command ran
after `terraform destory`.

terraform/Makefile

@@ -29,4 +29,4 @@ apply: preflight init ## Deploy the base infrastructure using terraform
 destroy: preflight init ## Tear down deployed the infrastructure
 	terraform destroy ${TERRAFORM_PARAMETERS}
 	# Cleanup remaining files
-	rm -rf .terraform .terraform terraform.tfstate terraform.tfstate.backup
+	rm -rf .terraform .terraform terraform.tfstate terraform.tfstate.backup terraform.tfstate.*.backup