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/controller/functionApi.go

@@ -339,7 +339,7 @@ func (a *API) FunctionPodLogs(w http.ResponseWriter, r *http.Request) {
 	}
 }
 
-func getContainerLog(kubernetesClient *kubernetes.Clientset, w http.ResponseWriter, fn *fv1.Function, pod *apiv1.Pod) error {
+func getContainerLog(kubernetesClient kubernetes.Interface, w http.ResponseWriter, fn *fv1.Function, pod *apiv1.Pod) error {
 	seq := strings.Repeat("=", 35)
 
 	for _, container := range pod.Spec.Containers {