TAS: validate that kubernetes.io/hostname can only be at the lowest level

[mimowo]

What would you like to be added:

validation to prevent setting kubernetes.io/hostname in the middle of hierarchy.

Why is this needed:

To guide user and avoid mis-configuration. This is particularly important because we have dedicated checks in our code, active only when kubernetes.io/hostname is at the lowest level.

[mimowo]

@tenzen-y @mbobrovskyi @PBundyra