Add delete image-policy and refactor

This adds a command for deleting ImagePolicy objects. Since the
control flow for the command needs only a runtime.Object (and a name
for the type), it can be factored out.

I have made the argument (field in the deleteCommand struct) an
interface `objectContainer`, through which the command code gets a
`runtime.Object` to deserialise into (and delete). It could be simply
a `runtime.Object` here; however things like `getCommand` require
other methods, so it's convenient to have an interface for it.

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

cmd/flux/object.go

@@ -0,0 +1,40 @@
+/*
+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 (
+	"k8s.io/apimachinery/pkg/runtime"
+)
+
+// objectContainer is an interface for a wrapper or alias from which we
+// can get a controller-runtime deserialisable value. This is used so
+// that you can wrap an API type to give it other useful methods, but
+// still use values of the wrapper with `client.Client`, which only
+// deals with types that have been added to the schema.
+type objectContainer interface {
+	AsClientObject() runtime.Object
+}
+
+// genericContainer is an objectContainer for any runtime.Object. Use
+// this if there are no other methods needed.
+type genericContainer struct {
+	obj runtime.Object
+}
+
+func (c genericContainer) AsClientObject() runtime.Object {
+	return c.obj
+}