tests: Replaces mounttest images used with agnhost (part 4)

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:

- mounttest
- mounttest-user

Additionally, removes the usage of the mounttest-user image and removes
it from kubernetes/test/images. RunAsUser is set instead of having that image.

test/e2e/common/util.go

@@ -64,8 +64,6 @@ var CommonImageWhiteList = sets.NewString(
 	imageutils.GetE2EImage(imageutils.Agnhost),
 	imageutils.GetE2EImage(imageutils.BusyBox),
 	imageutils.GetE2EImage(imageutils.IpcUtils),
-	imageutils.GetE2EImage(imageutils.Mounttest),
-	imageutils.GetE2EImage(imageutils.MounttestUser),
 	imageutils.GetE2EImage(imageutils.Nginx),
 	imageutils.GetE2EImage(imageutils.Httpd),
 	imageutils.GetE2EImage(imageutils.VolumeNFSServer),
@@ -74,17 +72,16 @@ var CommonImageWhiteList = sets.NewString(
 )
 
 type testImagesStruct struct {
-	AgnhostImage   string
-	BusyBoxImage   string
-	KittenImage    string
-	MounttestImage string
-	NautilusImage  string
-	NginxImage     string
-	NginxNewImage  string
-	HttpdImage     string
-	HttpdNewImage  string
-	PauseImage     string
-	RedisImage     string
+	AgnhostImage  string
+	BusyBoxImage  string
+	KittenImage   string
+	NautilusImage string
+	NginxImage    string
+	NginxNewImage string
+	HttpdImage    string
+	HttpdNewImage string
+	PauseImage    string
+	RedisImage    string
 }
 
 var testImages testImagesStruct
@@ -94,7 +91,6 @@ func init() {
 		imageutils.GetE2EImage(imageutils.Agnhost),
 		imageutils.GetE2EImage(imageutils.BusyBox),
 		imageutils.GetE2EImage(imageutils.Kitten),
-		imageutils.GetE2EImage(imageutils.Mounttest),
 		imageutils.GetE2EImage(imageutils.Nautilus),
 		imageutils.GetE2EImage(imageutils.Nginx),
 		imageutils.GetE2EImage(imageutils.NginxNew),