plugins: add support for customizations

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.

plugins/hub/osbuild.py

@@ -87,6 +87,10 @@ OSBUILD_IMAGE_SCHEMA = {
             "type": "object",
             "additionalProperties": False,
             "properties": {
+                "customizations": {
+                    "type": "object",
+                    "additionalProperties": True
+                },
                 "ostree": {
                     "type": "object",
                     "$ref": "#/definitions/ostree"