Add backend selection for agent (#463)

- add backend selection option
- by default it will auto-detect a backend

pipeline/backend/kubernetes/kubernetes.go

@@ -3,8 +3,9 @@ package kubernetes
 import (
 	"context"
 	"io"
+	"os"
 
-	"github.com/woodpecker-ci/woodpecker/pipeline/backend"
+	"github.com/woodpecker-ci/woodpecker/pipeline/backend/types"
 )
 
 type engine struct {
@@ -14,7 +15,7 @@ type engine struct {
 }
 
 // New returns a new Kubernetes Engine.
-func New(namespace, endpoint, token string) backend.Engine {
+func New(namespace, endpoint, token string) types.Engine {
 	return &engine{
 		namespace: namespace,
 		endpoint:  endpoint,
@@ -22,40 +23,53 @@ func New(namespace, endpoint, token string) backend.Engine {
 	}
 }
 
+func (e *engine) Name() string {
+	return "kubernetes"
+}
+
+func (e *engine) IsAvivable() bool {
+	host := os.Getenv("KUBERNETES_SERVICE_HOST")
+	return len(host) > 0
+}
+
+func (e *engine) Load() error {
+	return nil
+}
+
 // Setup the pipeline environment.
-func (e *engine) Setup(context.Context, *backend.Config) error {
+func (e *engine) Setup(context.Context, *types.Config) error {
 	// POST /api/v1/namespaces
 	return nil
 }
 
 // Start the pipeline step.
-func (e *engine) Exec(context.Context, *backend.Step) error {
+func (e *engine) Exec(context.Context, *types.Step) error {
 	// POST /api/v1/namespaces/{namespace}/pods
 	return nil
 }
 
 // DEPRECATED
 // Kill the pipeline step.
-func (e *engine) Kill(context.Context, *backend.Step) error {
+func (e *engine) Kill(context.Context, *types.Step) error {
 	return nil
 }
 
 // Wait for the pipeline step to complete and returns
 // the completion results.
-func (e *engine) Wait(context.Context, *backend.Step) (*backend.State, error) {
+func (e *engine) Wait(context.Context, *types.Step) (*types.State, error) {
 	// GET /api/v1/watch/namespaces/{namespace}/pods
 	// GET /api/v1/watch/namespaces/{namespace}/pods/{name}
 	return nil, nil
 }
 
 // Tail the pipeline step logs.
-func (e *engine) Tail(context.Context, *backend.Step) (io.ReadCloser, error) {
+func (e *engine) Tail(context.Context, *types.Step) (io.ReadCloser, error) {
 	// GET /api/v1/namespaces/{namespace}/pods/{name}/log
 	return nil, nil
 }
 
 // Destroy the pipeline environment.
-func (e *engine) Destroy(context.Context, *backend.Config) error {
+func (e *engine) Destroy(context.Context, *types.Config) error {
 	// DELETE /api/v1/namespaces/{name}
 	return nil
 }