kubeadm: Group centric component configs

kubeadm's current implementation of component config support is "kind" centric.
This has its downsides. Namely:
- Kind names and numbers can change between config versions.
  Newer kinds can be ignored. Therefore, detection of a version change is
  considerably harder.
- A component config can have only one kind that is managed by kubeadm.
Thus a more appropriate way to identify component configs is required.

Probably the best solution identified so far is a config group.
A group name is unlikely to change between versions, while the kind names and
structure can.
Tracking component configs by group name allows us to:
- Spot more easily config version changes and manage alternate versions.
- Support more than one kind in a config group/version.
- Abstract component configs by hiding their exact structure.

Hence, this change rips off the old kind based support for component configs
and replaces it with a group name based one. This also has the following
extra benefits:
- More tests were added.
- kubeadm now errors out if an unsupported version of a known component group
  is used.

Signed-off-by: Rostislav M. Georgiev <rostislavg@vmware.com>

cmd/kubeadm/app/componentconfigs/utils.go

@@ -0,0 +1,28 @@
+/*
+Copyright 2018 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 componentconfigs
+
+import (
+	"k8s.io/klog"
+)
+
+// warnDefaultComponentConfigValue prints a warning if the user modified a field in a certain
+// CompomentConfig from the default recommended value in kubeadm.
+func warnDefaultComponentConfigValue(componentConfigKind, paramName string, defaultValue, userValue interface{}) {
+	klog.Warningf("The recommended value for %q in %q is: %v; the provided value is: %v",
+		paramName, componentConfigKind, defaultValue, userValue)
+}