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>

docs/cmd/flux_delete_auto.md

@@ -26,5 +26,6 @@ The delete auto sub-commands delete automation objects.
 ### SEE ALSO
 
 * [flux delete](flux_delete.md)	 - Delete sources and resources
+* [flux delete auto image-policy](flux_delete_auto_image-policy.md)	 - Delete an ImagePolicy object
 * [flux delete auto image-repository](flux_delete_auto_image-repository.md)	 - Delete an ImageRepository object
 

docs/cmd/flux_delete_auto_image-policy.md

@@ -0,0 +1,41 @@
+## flux delete auto image-policy
+
+Delete an ImagePolicy object
+
+### Synopsis
+
+The delete auto image-policy command deletes the given ImagePolicy from the cluster.
+
+```
+flux delete auto image-policy [name] [flags]
+```
+
+### Examples
+
+```
+  # Delete an image policy
+  flux delete auto image-policy alpine3.x
+
+```
+
+### Options
+
+```
+  -h, --help   help for image-policy
+```
+
+### Options inherited from parent commands
+
+```
+      --context string      kubernetes context to use
+      --kubeconfig string   path to the kubeconfig file (default "~/.kube/config")
+  -n, --namespace string    the namespace scope for this operation (default "flux-system")
+  -s, --silent              delete resource without asking for confirmation
+      --timeout duration    timeout for this operation (default 5m0s)
+      --verbose             print generated objects
+```
+
+### SEE ALSO
+
+* [flux delete auto](flux_delete_auto.md)	 - Delete automation objects
+