Provide arm64 images for M1/M2 Macs

[jmbowman]

There were two previous attempts (#852 and #920) to build devstack images that could run natively on newer ARM-based Apple computers, but both ran into significant problems trying to get working Ansible builds on that architecture. Thanks to the work in #943, we no longer use Ansible to build our devstack images. We should attempt again to build arm64 versions of all our devstack images; Tutor already does this, so it should be feasible (and its Dockerfile templates may serve as a useful reference if problems are encountered).