Use Clientset interface instead of type for Fission/kubernetes clients (#2416)

Using interface makes it easy to create a fake client and unit test
a specific portion of the code. We should be able to more write unit
test and increase coverage of code with this change.

Signed-off-by: Sanket Sudake <sanketsudake@gmail.com>

cmd/preupgradechecks/checks.go

@@ -32,14 +32,15 @@ import (
 
 	fv1 "github.com/fission/fission/pkg/apis/core/v1"
 	"github.com/fission/fission/pkg/crd"
+	"github.com/fission/fission/pkg/generated/clientset/versioned"
 )
 
 type (
 	PreUpgradeTaskClient struct {
 		logger        *zap.Logger
-		fissionClient *crd.FissionClient
-		k8sClient     *kubernetes.Clientset
-		apiExtClient  *apiextensionsclient.Clientset
+		fissionClient versioned.Interface
+		k8sClient     kubernetes.Interface
+		apiExtClient  apiextensionsclient.Interface
 		fnPodNs       string
 		envBuilderNs  string
 	}