Image alias support for the constraint system. As per discussion at the
meeting, we use the image aliases to create an equivalence class for the hardware types the image alias refers to. For example, UBUNTU16-64-STD is an x86 image in the image server. It is also an image alias (at the Moonshot cluster) that points to the x86 image and the ARM image. So that means UBUNTU16-64-STD runs on x86 types and moonshot types.