CI: tell Nova which CPU to emulate on AArch64

AArch64 does not have a way to get cpu features from libvirt.

Change-Id: Ieed404e17e8a9829f38c03f7ee7fdb3caa3919e8

tests/templates/nova-compute-overrides.j2

 [libvirt]
 virt_type=qemu
 # NOTE(Jeffrey4l): fix the gate in iax-ord nodes for libvirt 2.0.
+{% if ansible_architecture == 'aarch64' %}
+cpu_mode=custom
+cpu_model=max
+{% else %}
 cpu_mode=none
+{% endif %}
 
 {# TODO(mnasiadka): Remove in Victoria #}
 {% if scenario == 'ceph-ansible' and is_previous_release %}