napalm_device_types' create_vm_from_cloud_init gained cpu_type, the CPU model
a new VM gets (netork#494). VMware sets CPU compatibility per cluster (EVC),
not per VM, and this driver has no get_vm_cpu_types() to offer a model, so
None is the only value it can honour. Anything else raises ValueError before
the image is fetched rather than being dropped silently.