feat: specify api microversion for nova client in magnum conf (#1822)

2.15 is minimum version to support soft-* server group policy rule

Reviewed-by: Mohammed Naser <mnaser@vexxhost.com>
diff --git a/roles/magnum/vars/main.yml b/roles/magnum/vars/main.yml
index 99ef849..78884fe 100644
--- a/roles/magnum/vars/main.yml
+++ b/roles/magnum/vars/main.yml
@@ -69,6 +69,7 @@
         endpoint_type: internalURL
         region_name: "{{ openstack_helm_endpoints_neutron_region_name }}"
       nova_client:
+        api_version: 2.15
         endpoint_type: internalURL
         region_name: "{{ openstack_helm_endpoints_nova_region_name }}"
       octavia_client: