Add an ARCH parameter to handle arch specific things, mostly are the
docker image repo names.
Because not all the docker images magnum used support multi-arch
manifest[1] like kubernetes-dashboard, it will need to specific the
arch name in the docker image repo name.
[1]
https://kubernetes.io/docs/concepts/containers/images/#building-multi-architecture-images-with-manifests
Change-Id: Iccb3a030aefd2d4e55a455d1a0401cbc4eb7fd14
Task: 37884
Story: 2007026