Support building an allow-list of providers (#527)

* Add providers subcommand to verify providers

Allows users to check what providers are available

* Fix version output to add new line

This command was totally broken until we moved around the call to
`initConfig()`, this just fixes the output now that it works.

* Flip boolean of provider include tags

All providers are still included by default and fix tags using the old
format.

cmd/providers.go

@@ -0,0 +1,55 @@
+// Copyright © 2017 NAME HERE <EMAIL ADDRESS>
+//
+// 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 cmd
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+	"github.com/virtual-kubelet/virtual-kubelet/providers/register"
+)
+
+// versionCmd represents the version command
+var providersCmd = &cobra.Command{
+	Use:   "providers",
+	Short: "Show the list of supported providers",
+	Long:  "Show the list of supported providers",
+	Args:  cobra.MaximumNArgs(2),
+	Run: func(cmd *cobra.Command, args []string) {
+		switch len(args) {
+		case 0:
+			ls := register.List()
+			for _, p := range ls {
+				fmt.Fprintln(cmd.OutOrStdout(), p)
+			}
+		case 1:
+			if !register.Exists(args[0]) {
+				fmt.Fprintln(cmd.OutOrStderr(), "no such provider", args[0])
+
+				// TODO(@cpuuy83): would be nice to not short-circuit the exit here
+				// But at the momemt this seems to be the only way to exit non-zero and
+				// handle our own error output
+				os.Exit(1)
+			}
+			fmt.Fprintln(cmd.OutOrStdout(), args[0])
+		}
+		return
+	},
+}
+
+func init() {
+	RootCmd.AddCommand(providersCmd)
+}