refactor sample 6 (kubevela#63)

Signed-off-by: roy wang <[email protected]>

6.Knative_App/README.md

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+# Integrate Knative into KubeVela
+
+These demonstrations show how KubeVela integrates Knative Configuration/Route to
+deploy and manage applications.
+We will deploy an application with Knative Serving Workload, then interact with
+it using cURL requests.
+
+> The demonstrations are verified in Alibaba Kubernetes Cluster v1.18.8 of
+> Hongkong region.
+
+## Prerequisites
+
+* Follow the instructions in the
+[installation](https://kubevela.io/docs/install) document to get KubeVela
+installed.
+
+* Knative Serving v0.21.
+Refer to [Install
+Knative](https://knative.dev/docs/install/any-kubernetes-cluster/) to install
+Knative. Remember to:
+    * pick `Istio` as the networking layer 
+    * choose `Magic DNS (xip.io)` to configure DNS.
+
+## Quick Start
+
+### Prepare ComponentDefinition and Application
+
+We need an ComponentDefinition capable of renderring a `Service` of Knative
+`serving.knative.dev/v1`.
+
+```yaml
+apiVersion: core.oam.dev/v1beta1
+kind: ComponentDefinition
+metadata:
+  name: knative-serving
+  annotations:
+    definition.oam.dev/description: "Knative serving."
+spec:
+  workload:
+    definition:
+      apiVersion: serving.knative.dev/v1
+      kind: Service
+  schematic:
+    cue:
+      template: |
+        output: {
+        	apiVersion: "serving.knative.dev/v1"
+        	kind:       "Service"
+        	spec: {
+        		template:
+        			spec:
+        				containers: [{
+        					image: parameter.image
+        					env:   parameter.env
+        				}]
+        	}
+        }
+        parameter: {
+        	image: string
+        	env?: [...{
+        		// +usage=Environment variable name
+        		name: string
+        		// +usage=The value of the environment variable
+        		value?: string
+        		// +usage=Specifies a source the value of this var should come from
+        		valueFrom?: {
+        			// +usage=Selects a key of a secret in the pod's namespace
+        			secretKeyRef: {
+        				// +usage=The name of the secret in the pod's namespace to select from
+        				name: string
+        				// +usage=The key of the secret to select from. Must be a valid secret key
+        				key: string
+        			}
+        		}
+        	}]
+        }
+```
+
+```shell
+$ kubectl apply -f ./componentdefinition-knative-serving.yaml
+```
+
+Then we write an application with the newly created ComponentDefinition.
+
+```yaml
+apiVersion: core.oam.dev/v1beta1
+kind: Application
+metadata:
+  name: webapp
+spec:
+  components:
+    - name: backend
+      type: knative-serving
+      properties:
+        image: gcr.io/knative-samples/helloworld-go
+        env:
+          - name: TARGET
+            value: "Go Sample v1"
+```
+
+```shell
+$ kubectl apply -f ./app.yaml
+```
+
+### Verify the application
+
+```shell
+$ kubectl get application
+NAME     AGE
+webapp   24m
+
+$ kubectl get ksvc
+NAME            URL                                                 LATESTCREATED         LATESTREADY           READY   REASON
+backend-v1      http://backend-v1.default.47.242.55.215.xip.io      backend-v1-00001      backend-v1-00001      True
+
+$ curl http://backend-v1.default.47.242.55.215.xip.io
+Hello Go Sample v1!
+```
+
+Have fun with KubeVela and Knative.

6.Knative_App/app.yaml

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+apiVersion: core.oam.dev/v1beta1
+kind: Application
+metadata:
+  name: webapp
+spec:
+  components:
+    - name: backend
+      type: knative-serving
+      properties:
+        image: gcr.io/knative-samples/helloworld-go
+        env:
+          - name: TARGET
+            value: "Go Sample v1"

6.Knative_App/componentdefinition-knative-serving.yaml

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+apiVersion: core.oam.dev/v1beta1
+kind: ComponentDefinition
+metadata:
+  name: knative-serving
+  annotations:
+    definition.oam.dev/description: "Knative serving."
+spec:
+  workload:
+    definition:
+      apiVersion: serving.knative.dev/v1
+      kind: Service
+  schematic:
+    cue:
+      template: |
+        output: {
+        	apiVersion: "serving.knative.dev/v1"
+        	kind:       "Service"
+        	spec: {
+        		template:
+        			spec:
+        				containers: [{
+        					image: parameter.image
+        					env:   parameter.env
+        				}]
+        	}
+        }
+        parameter: {
+        	image: string
+        	env?: [...{
+        		// +usage=Environment variable name
+        		name: string
+        		// +usage=The value of the environment variable
+        		value?: string
+        		// +usage=Specifies a source the value of this var should come from
+        		valueFrom?: {
+        			// +usage=Selects a key of a secret in the pod's namespace
+        			secretKeyRef: {
+        				// +usage=The name of the secret in the pod's namespace to select from
+        				name: string
+        				// +usage=The key of the secret to select from. Must be a valid secret key
+        				key: string
+        			}
+        		}
+        	}]
+        }