Add dev/stg/prod environments with per-env values, configmaps and secrets

- Move env-specific config into environments/<env>/{values,configmap,secret}.yaml
  for both nginx-chart and mysql-chart (dev, stg, prod)
- Make templates env-agnostic: resource names come from values
  (configMap.name, secret.name)
- nginx deployment mounts env ConfigMap and loads env Secret via envFrom
- mysql statefulset reads DB config from ConfigMap, passwords from Secret
- Remove mysql-chart/templates/secret.yaml (secrets are now per-env manifests)
- Add README with layout and deploy commands

nginx-chart/environments/stg/configmap.yaml

@@ -0,0 +1,19 @@
+apiVersion: v1
+kind: ConfigMap
+metadata:
+  name: nginx-config-stg
+  namespace: stg
+  labels:
+    app: nginx
+    env: stg
+data:
+  default.conf: |
+    server {
+        listen 80;
+        server_name nginx.stg.local;
+
+        location / {
+            return 200 "nginx STG environment\n";
+            add_header Content-Type text/plain;
+        }
+    }