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>
