Clarify API validation error if operator is Exists

Without this patch the error message for this example:

```
---
apiVersion: v1
kind: Pod
metadata:
  name: test
spec:
  containers:
    - name: agent
      image: debian:latest
  tolerations:
    - key: pool
      operator: Exists
      value: build
      effect: NoSchedule
```

Looks like:

```
The Pod "test" is invalid: spec.tolerations[0].operator: Invalid value:
core.Toleration{Key:"pool", Operator:"Exists", Value:"build",
Effect:"NoSchedule", TolerationSeconds:(*int64)(nil)}: value must be
empty when `operator` is 'Exists'
```

To clarify that the `Value` field is wrong, we now directly point the
`field.Invalid` to it. Now the error message becomes a more clear and
concise one:

```
The Pod "test" is invalid: spec.tolerations[0].operator: Invalid value:
"build": value must be empty when `operator` is 'Exists'
```

Signed-off-by: Sascha Grunert <[email protected]>

Author: saschagrunert

pkg/apis/core/validation/validation.go

@@ -3990,7 +3990,7 @@ func ValidateTolerations(tolerations []core.Toleration, fldPath *field.Path) fie
 			}
 		case core.TolerationOpExists:
 			if len(toleration.Value) > 0 {
-				allErrors = append(allErrors, field.Invalid(idxPath.Child("operator"), toleration, "value must be empty when `operator` is 'Exists'"))
+				allErrors = append(allErrors, field.Invalid(idxPath.Child("operator"), toleration.Value, "value must be empty when `operator` is 'Exists'"))
 			}
 		default:
 			validValues := []core.TolerationOperator{core.TolerationOpEqual, core.TolerationOpExists}