Remove kubectl's dependence on schema file in pkg/api/validation.

**What this PR does / why we need it**:

Makes functions in validation/schema.go private to kubectl,
further isolating kubectl.

**Which issue this PR fixes**

Part of a series of PRs to address kubernetes/community#598

**Release note**:
```release-note
NONE
```

pkg/api/validation/validation_test.go

@@ -32,6 +32,7 @@ import (
 	utilfeature "k8s.io/apiserver/pkg/util/feature"
 	"k8s.io/kubernetes/pkg/api"
 	"k8s.io/kubernetes/pkg/api/helper"
+	_ "k8s.io/kubernetes/pkg/api/testapi"
 	"k8s.io/kubernetes/pkg/capabilities"
 	"k8s.io/kubernetes/pkg/security/apparmor"
 )