Your flavor looks correct. It's just that you don't have any hypervisors (nova-compute nodes) that can run instances with the desired ppc architecture. Try configuring (some of) your compute nodes with cpu_mode set to custom and an appropriate cpu_model in the [libvirt]section of nova.conf. We do this here to emulate slightly different (restricted) models of Intel x86 CPUs compared to the actual hardware; I'm not sure whether this mechanism is general enough to select a different CPU architecture.