Add the image_copy plugin

Image_copy supports creation of dirs and copying of files
from the host system into the image.

bug #168
You can reuse the `absolute_path` definition from the [main manifest schema](https://github.com/andsens/bootstrap-vz/blob/4c315f80b7923666d2da4061cc1bb2705035d0d9/bootstrapvz/base/manifest-schema.yml#L140).
Just replace `type: string` with `$ref: '#/definitions/absolute_path'`.