kubeadm: conditionally set the kubelet cgroup driver for Docker

Add a new utility file - util/cgroupdriver.go.
Currently it only contains the public function GetCgroupDriverDocker().
The function uses 'docker info' to obtain the cgroup driver
for Docker.

On a later stage this file can contain more methods for different
CRI.

Use GetCgroupDriverDocker() in phases/kubelet/flags.go
to conditionally set the 'cgroup-driver' argument. On error
print a warning and don't set the argument value.

Add unit tests in cgroupdriver_test.go.

cmd/kubeadm/app/util/cgroupdriver_test.go

@@ -0,0 +1,68 @@
+/*
+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 util
+
+import (
+	"testing"
+)
+
+func TestGetCgroupDriverDocker(t *testing.T) {
+	testCases := []struct {
+		name          string
+		info          string
+		expectedError bool
+	}{
+		{
+			name:          "valid: value is 'cgroupfs'",
+			info:          `Cgroup Driver: cgroupfs`,
+			expectedError: false,
+		},
+		{
+			name:          "valid: value is 'systemd'",
+			info:          `Cgroup Driver: systemd`,
+			expectedError: false,
+		},
+		{
+			name:          "invalid: missing 'Cgroup Driver' key and value",
+			info:          "",
+			expectedError: true,
+		},
+		{
+			name:          "invalid: only a 'Cgroup Driver' key is present",
+			info:          `Cgroup Driver`,
+			expectedError: true,
+		},
+		{
+			name:          "invalid: empty 'Cgroup Driver' value",
+			info:          `Cgroup Driver: `,
+			expectedError: true,
+		},
+		{
+			name:          "invalid: unknown 'Cgroup Driver' value",
+			info:          `Cgroup Driver: invalid-value`,
+			expectedError: true,
+		},
+	}
+
+	for _, tc := range testCases {
+		t.Run(tc.name, func(t *testing.T) {
+			if _, err := getCgroupDriverFromDockerInfo(tc.info); (err != nil) != tc.expectedError {
+				t.Fatalf("expected error: %v, saw: %v, error: %v", tc.expectedError, (err != nil), err)
+			}
+		})
+	}
+}