Fix: Make the out flag print to one file using seperator (#1541)

* fix: make the out flag print to one file using seperator

* fix: comment an unused function

* fix: update test script

* fix: update output k8s

* fix: update the json output error condition

* fix: update envvars interpolation tests

* chore: update e2e tests to support yaml generation instead of JSON format

* fix: update e2e tests according to ci environment

* fix: apply PR changes

script/test/fixtures/single-file-output/docker-compose.yaml

@@ -0,0 +1,12 @@
+version: "3.8"
+
+services:
+  front_end:
+    image: gcr.io/google-samples/gb-frontend:v4
+    ports:
+      - "80:80"
+    environment:
+      - GET_HOSTS_FROM=dns
+    labels:
+      kompose.service.expose: lb
+      kompose.service.expose.ingress-class-name: nginx