Change the dashboard and heapster images in k8s to arm64 when running on arm platform

Change the dashboard and heapster images in k8s to arm64 when running on arm platform

Change the dashboard image in k8s from amd64 to arm64 on arm platform.
In extra-vars-aarch64.yml, add the dashboard image tag for supporting the
arm64 arch.

For heapster images, add a "sed" command in main.yml for replacing
the cpu architecture from amd64 to arm64.
And it will be invoked only when deploying the arm platform.

Change-Id: I584485f0356dcb925814f63cbc45ecbd459715bb
Signed-off-by: Jingzhao Ni <jingzhao.ni@arm.com>