Add controller API client interface (#1467)

This PR adds an interface for controller API client, it allows us to
implement mock API client for unit testing with ease and we are
able to generate spec file without accessing the real Fission server.

pkg/fission-cli/cmd/spec/destroy.go

@@ -19,25 +19,18 @@ package spec
 import (
 	"github.com/pkg/errors"
 
-	"github.com/fission/fission/pkg/controller/client"
 	"github.com/fission/fission/pkg/fission-cli/cliwrapper/cli"
+	"github.com/fission/fission/pkg/fission-cli/cmd"
 	"github.com/fission/fission/pkg/fission-cli/util"
 )
 
 type DestroySubCommand struct {
-	client *client.Client
+	cmd.CommandActioner
 }
 
 // Destroy destroys everything in the spec.
 func Destroy(input cli.Input) error {
-	c, err := util.GetServer(input)
-	if err != nil {
-		return err
-	}
-	opts := &DestroySubCommand{
-		client: c,
-	}
-	return opts.do(input)
+	return (&DestroySubCommand{}).do(input)
 }
 
 func (opts *DestroySubCommand) do(input cli.Input) error {
@@ -59,7 +52,7 @@ func (opts *DestroySubCommand) run(input cli.Input) error {
 	emptyFr.DeploymentConfig = fr.DeploymentConfig
 
 	// "apply" the empty state
-	_, _, err = applyResources(opts.client, specDir, &emptyFr, true)
+	_, _, err = applyResources(opts.Client(), specDir, &emptyFr, true)
 	if err != nil {
 		return errors.Wrap(err, "error deleting resources")
 	}