Kubelet: Relative paths in local config file

Resolve relative paths against the config file's location.
Add a test to help keep the list of filepath fields up to date.

Issue: #55644
Related comment:
https://github.com/kubernetes/kubernetes/pull/53833#issuecomment-344009912

Will add the same behavior for dynamic Kubelet config in a future PR,
see issue #55645.

pkg/kubelet/apis/kubeletconfig/helpers.go

@@ -0,0 +1,34 @@
+/*
+Copyright 2017 The Kubernetes Authors.
+
+Licensed under the Apache License, Version 2.0 (the "License");
+you may not use this file except in compliance with the License.
+You may obtain a copy of the License at
+
+    http://www.apache.org/licenses/LICENSE-2.0
+
+Unless required by applicable law or agreed to in writing, software
+distributed under the License is distributed on an "AS IS" BASIS,
+WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+See the License for the specific language governing permissions and
+limitations under the License.
+*/
+
+package kubeletconfig
+
+// KubeletConfigurationPathRefs returns pointers to all of the KubeletConfiguration fields that contain filepaths.
+// You might use this, for example, to resolve all relative paths against some common root before
+// passing the configuration to the application. This method must be kept up to date as new fields are added.
+func KubeletConfigurationPathRefs(kc *KubeletConfiguration) []*string {
+	paths := []*string{}
+	paths = append(paths, &kc.PodManifestPath)
+	paths = append(paths, &kc.Authentication.X509.ClientCAFile)
+	paths = append(paths, &kc.TLSCertFile)
+	paths = append(paths, &kc.TLSPrivateKeyFile)
+	paths = append(paths, &kc.SeccompProfileRoot)
+	paths = append(paths, &kc.ResolverConfig)
+	// TODO(#55562): planning on moving two out of KubeletConfiguration
+	paths = append(paths, &kc.VolumePluginDir)
+	paths = append(paths, &kc.LockFilePath)
+	return paths
+}