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>
This uses the established abstractions to implement the usual
subcommands for the ImageUpdateAutomation type.
I've called the sub-subcommand in each case `image-update`, as a
fairly safe shorthand for the much longer `image-update-automation`.
Signed-off-by: Michael Bridgen <michael@weave.works>
