Add ability to consume secrets in env vars

docs/user-guide/secrets/secret-env-pod.yaml

@@ -0,0 +1,16 @@
+apiVersion: v1
+kind: Pod
+metadata:
+  name: secret-env-pod
+spec:
+  containers:
+    - name: test-container
+      image: gcr.io/google_containers/busybox
+      command: [ "/bin/sh", "-c", "env" ]
+      env:
+        - name: MY_SECRET_DATA
+          valueFrom:
+            secretKeyRef:
+              name: test-secret
+              key: data-1
+  restartPolicy: Never