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/healthcheck/healthcheck.go

@@ -62,7 +62,7 @@ type HealthChecker struct {
 	categories []*Category
 	*Options
 
-	kubeAPI          *kubernetes.Clientset
+	kubeAPI          kubernetes.Interface
 	fissionNamespace string
 }
 
@@ -84,7 +84,7 @@ func isCompatibleVersion(minimalRequirementVersion [3]int, actualVersion [3]int)
 
 func (hc *HealthChecker) CheckKubeVersion() (err error) {
 
-	version, err := hc.kubeAPI.ServerVersion()
+	version, err := hc.kubeAPI.Discovery().ServerVersion()
 	if err != nil {
 		return err
 	}