Expose the Libvirt cpu_model and cpu_model_extra_flags options

Being able to set cpu_mode to "custom" is nice but doesn't do us
much good if users then have to resort to nova_nova_conf_overrides
to set cpu_model and/or cpu_model_extra_flags.

Instead, honor nova_cpu_model and nova_cpu_model_extra_flags variable.
Also add a release note saying that for those users who did previously
set these flags via nova_nova_conf_overrides, they should move
those bits to the newly understood variables.

Change-Id: I889e70826c5c631b86305b512b86fba2b89521b4

releasenotes/notes/nova-cpu-model-006da20048168842.yaml

@@ -0,0 +1,18 @@
+---
+features:
+  - You can now set the Libvirt CPU model and feature flags from the
+    appropriate entry under the ``nova_virt_types`` dictionary variable
+    (normally ``kvm``).
+    ``nova_cpu_model`` is a string value that sets the CPU model; this
+    value is ignored if you set any ``nova_cpu_mode`` other than
+    ``custom``.
+    ``nova_cpu_model_extra_flags`` is a list that allows you to specify
+    extra CPU feature flags not normally passed through with
+    ``host-model``, or the ``custom`` CPU model of your choice.
+upgrade:
+  - If your configuration previously set the ``libvirt/cpu_model``
+    and/or ``libvirt/cpu_model_extra_flags`` variables in a
+    ``nova_nova_conf_overrides`` dictionary, you should consider
+    moving those to ``nova_cpu_model`` and
+    ``nova_cpu_model_extra_flags`` in the appropriate entry (normally
+    ``kvm``) in the ``nova_virt_types`` dictionary.

templates/nova.conf.j2

@@ -258,6 +258,12 @@ virt_type = {{ nova_virt_type }}
 remove_unused_resized_minimum_age_seconds = {{ nova_remove_unused_resized_minimum_age_seconds }}
 {% if nova_cpu_mode is defined %}
 cpu_mode = {{ nova_cpu_mode }}
+{% if nova_cpu_mode == 'custom' %}
+cpu_model = {{ nova_cpu_model }}
+{% endif %}
+{% if nova_cpu_model_extra_flags is defined %}
+cpu_model_extra_flags = {{ nova_cpu_model_extra_flags | join(',') }}
+{% endif %}
 {% endif %}
 
 {% if nova_libvirt_images_rbd_pool is defined %}