If the `AdditionalArchitectures` parameter has entries then the
container image prepare will prepare images for all architectures
instead of just the default one. A new boolean field `multi_arch` can
also be set in `ContainerImagePrepare` entries to determine the multi
arch behaviour for images in that entry. If any entry sets a
`multi_arch` value then `AdditionalArchitectures` is ignored.
This change defines the interface for supporting multi-arch prepare
operations, a later change in this series will implement the support.
Blueprint: multiarch-support
Change-Id: Ia7b33ab6bec0fabc8632e58807e749b6e2a8c9a3