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>

pkg/fission-cli/util/util.go

@@ -105,7 +105,7 @@ func KubifyName(old string) string {
 // GetKubernetesClient builds a new kubernetes client. If the KUBECONFIG
 // environment variable is empty or doesn't exist, ~/.kube/config is used for
 // the kube config path
-func GetKubernetesClient(kubeContext string) (*restclient.Config, *kubernetes.Clientset, error) {
+func GetKubernetesClient(kubeContext string) (*restclient.Config, kubernetes.Interface, error) {
 	loadingRules := clientcmd.NewDefaultClientConfigLoadingRules()
 
 	kubeConfigPath := os.Getenv("KUBECONFIG")