The Cloud API supports passing in a variety of image customizations,
like e.g. extra packages or pre-defining users.
Add a new command line option to the client `--customizations` which
takes a path to a JSON file which contains the customziations; they
will be passed via the existing `opts` argument to the hub.
Add support for `customizations` to the `opts`/`options` arguments
to the hub plugin. No validation to the object is done. Instead we
rely in Composer for the validation of the content.
Add support for `customizations` the image `ComposeRequest` in the
builder plugin. All specified values are just passed through to
composer as-is.
Add tests for the respective plugins.
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