tests: Replaces images used with agnhost (part 2)

Quite a few images are only used a few times in a few tests. Thus,
the images are being centralized into the agnhost image, reducing
the number of images that have to be pulled and used.

This PR replaces the usage of the following images with agnhost:

- net
- netexec
- nettest
- webhook

test/e2e/node/pre_stop.go

@@ -51,7 +51,8 @@ func testPreStop(c clientset.Interface, ns string) {
 			Containers: []v1.Container{
 				{
 					Name:  "server",
-					Image: imageutils.GetE2EImage(imageutils.Nettest),
+					Image: imageutils.GetE2EImage(imageutils.Agnhost),
+					Args:  []string{"nettest"},
 					Ports: []v1.ContainerPort{{ContainerPort: 8080}},
 				},
 			},