add support for non-amd64 arch gcr.io images

Currently all the gcr.io images used in kubespray can only run on x86.
Also gcr.io has not fully support multi-arch docker images.

Add extra var "image_arch" (default is amd64) to support running other
platforms, like arm64.

Change-Id: I8e1c9af533c021cb96ade291a1ce58773b40e271