Add terraform fmt to terraform_lint

@dcarley has implemented new canonical formatting functionality, which has
been accepted to terraform (hashicorp/hcl#80). Use
the new command to check if any files need formatting.

Makefile

@@ -48,6 +48,7 @@ lint_terraform: dev
 	$(eval export TF_VAR_system_dns_zone_name=$SYSTEM_DNS_ZONE_NAME)
 	$(eval export TF_VAR_apps_dns_zone_name=$APPS_DNS_ZONE_NAME)
 	find terraform -mindepth 1 -maxdepth 1 -type d -not -path 'terraform/providers' -not -path 'terraform/scripts' -print0 | xargs -0 -n 1 -t terraform graph > /dev/null
+	@if [ "$$(terraform fmt -write=false terraform)" != "" ] ; then echo "Use 'terraform fmt' to fix HCL formatting:"; terraform fmt -write=false -diff=true terraform ; exit 1; fi
 
 lint_shellcheck:
 	find . -name '*.sh' -not -path '*/vendor/*' | xargs $(SHELLCHECK)