Rename "auto" subcommands to "image"

This means all the sub-subcommands can drop the `image-` prefix,
making them shorter and more fluent.

E.g.,

    flux create image policy

rather than

    flux create auto image-policy

Signed-off-by: Michael Bridgen <michael@weave.works>

cmd/flux/delete_image.go

@@ -0,0 +1,31 @@
+/*
+Copyright 2020 The Flux 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 main
+
+import (
+	"github.com/spf13/cobra"
+)
+
+var deleteAutoCmd = &cobra.Command{
+	Use:   "auto",
+	Short: "Delete automation objects",
+	Long:  "The delete auto sub-commands delete automation objects.",
+}
+
+func init() {
+	deleteCmd.AddCommand(deleteAutoCmd)
+}