Add ARM64 overcloud host image builds

[cityofships]

An alternative to #1497.

• Don't give a choice to operators for picking a platform - images targeting both platforms are always built and uploaded, so they stay close to each other.
• Make distro and platform builds parallel so the workflow finishes faster.
• Does not use cross-arch at all. Leafcloud builds will not be supported, since there's no arm64 hypervisors there.

Depends on: stackhpc/stackhpc-release-train#441