Refactor CLI initialization (#562)

This cleans up the CLI code significantly.
Also makes some of this re-usable for providers who want to do so.

This also removes the main.go from the top of the tree of the repro,
instead moving it into cmd/virtual-kubelet.
This allows us to better utilize the package namespace (and e.g. mv the
`vkubelet` package to the top of the tree).

virtual-kubelet/commands/providers/provider.go

@@ -0,0 +1,54 @@
+// Copyright © 2017 The virtual-kubelet 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 providers
+
+import (
+	"fmt"
+	"os"
+
+	"github.com/spf13/cobra"
+	"github.com/virtual-kubelet/virtual-kubelet/providers/register"
+)
+
+// NewCommand creates a new providers subcommand
+// This subcommand is used to determine which providers are registered.
+func NewCommand() *cobra.Command {
+	return &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
+		},
+	}
+}