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.