Use narrowly scoped interfaces for client access

Use custom narrowly scoped interfaces for client access from the
RollingUpdater and Resizer. This allows for more flexible downstream
integration and unit testing without imposing a burden to implement
the entire client.Interface for just a handful of methods.

pkg/kubectl/rolling_updater_test.go

@@ -253,7 +253,7 @@ Update succeeded. Deleting foo-v1
 
 	for _, test := range tests {
 		updater := RollingUpdater{
-			fakeClientFor("default", test.responses),
+			&RealRollingUpdaterClient{fakeClientFor("default", test.responses)},
 			"default",
 		}
 		var buffer bytes.Buffer
@@ -296,7 +296,7 @@ Update succeeded. Deleting foo-v1
 		{newRc(3, 3), nil},
 		{newRc(3, 3), nil},
 	}
-	updater := RollingUpdater{fakeClientFor("default", responses), "default"}
+	updater := RollingUpdater{&RealRollingUpdaterClient{fakeClientFor("default", responses)}, "default"}
 
 	var buffer bytes.Buffer
 	if err := updater.Update(&buffer, rc, rcExisting, 0, time.Millisecond, time.Millisecond); err != nil {