Implement `init` subcommand

This commit will implement the first version of the `init` subcommand.
This subcommand initializes a ksonnet application, including generating
the default directory tree and generating the appropriate ksonnet-lib.

cmd/init.go

+// Copyright 2017 The kubecfg 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 cmd
+
+import (
+	"fmt"
+	"os"
+	"path"
+	"path/filepath"
+
+	"github.com/ksonnet/kubecfg/metadata"
+	"github.com/ksonnet/kubecfg/pkg/kubecfg"
+	"github.com/spf13/cobra"
+)
+
+const (
+	flagAPISpec = "api-spec"
+)
+
+func init() {
+	RootCmd.AddCommand(initCmd)
+	// TODO: We need to make this default to checking the `kubeconfig` file.
+	initCmd.PersistentFlags().String(flagAPISpec, "version:v1.7.0", "Manually specify API version from OpenAPI schema, cluster, or Kubernetes version")
+}
+
+var initCmd = &cobra.Command{
+	Use:   "init <app-name>",
+	Short: "Initialize a ksonnet project",
+	RunE: func(cmd *cobra.Command, args []string) error {
+		flags := cmd.Flags()
+		if len(args) != 1 {
+			return fmt.Errorf("'init' takes a single argument that names the application we're initializing")
+		}
+
+		appName := args[0]
+		appDir, err := filepath.Abs(filepath.Dir(os.Args[0]))
+		if err != nil {
+			return err
+		}
+		appRoot := metadata.AbsPath(path.Join(appDir, appName))
+
+		specFlag, err := flags.GetString(flagAPISpec)
+		if err != nil {
+			return err
+		}
+
+		c, err := kubecfg.NewInitCmd(appRoot, specFlag)
+		if err != nil {
+			return err
+		}
+
+		return c.Run()
+	},
+	Long: `Initialize a ksonnet project in a new directory, 'app-name'. This process
+consists of two steps:
+
+1. Generating ksonnet-lib. Users can set flags to generate the library based on
+   a variety of data, including server configuration and an OpenAPI
+   specification of a Kubernetes build. By default, this is generated from the
+   capabilities of the cluster specified in the cluster of the current context
+   specified in $KUBECONFIG.
+2. Generating the following tree in the current directory.
+
+   app-name/
+     .gitignore     Default .gitignore; can customize VCS
+     .ksonnet/      Metadata for ksonnet
+     envs/          Env specs (defaults: dev, test, prod)
+       params.yaml  Specifies the schema of the environments
+       dev.yaml
+       test.yaml
+       prod.yaml
+       us-east.yaml [Example of user-generated env]
+     components/    Top-level Kubernetes objects defining application
+     lib/           user-written .libsonnet files
+     vendor/        mixin libraries, prototypes
+`,
+	Example: `  # Initialize ksonnet application, using the capabilities of live cluster
+  # specified in the $KUBECONFIG environment variable (specifically: the
+  # current context) to generate 'ksonnet-lib'.
+  ksonnet init app-name
+
+  # Initialize ksonnet application, using the OpenAPI specification generated
+  # in the Kubenetes v1.7.1 build to generate 'ksonnet-lib'.
+  ksonnet init app-name --api-spec=version:v1.7.1
+
+  # Initialize ksonnet application, using an OpenAPI specification file
+  # generated by a build of Kubernetes to generate 'ksonnet-lib'.
+  ksonnet init app-name --api-spec=file:swagger.json`,
+}

pkg/kubecfg/init.go

+package kubecfg
+
+import "github.com/ksonnet/kubecfg/metadata"
+
+type InitCmd struct {
+	rootPath metadata.AbsPath
+	spec     metadata.ClusterSpec
+}
+
+func NewInitCmd(rootPath metadata.AbsPath, specFlag string) (*InitCmd, error) {
+	// NOTE: We're taking `rootPath` here as an absolute path (rather than a partial path we expand to an absolute path)
+	// to make it more testable.
+
+	spec, err := metadata.ParseClusterSpec(specFlag)
+	if err != nil {
+		return nil, err
+	}
+
+	return &InitCmd{rootPath: rootPath, spec: spec}, nil
+}
+
+func (c *InitCmd) Run() error {
+	_, err := metadata.Init(c.rootPath, c.spec)
+	return err
+}