format pxe_append_params as string and not a list

Please see: https://docs.openstack.org/ironic/latest/configuration/sample-config.html

Change-Id: Idc96523a5108712a072b14889b1fba0f05c381e8
Story: 2006707
Task: 37067

ansible/roles/kolla-openstack/templates/ironic.conf.j2

@@ -29,7 +29,7 @@ provisioning_network = {{ kolla_ironic_provisioning_network }}
 
 [pxe]
 {% if kolla_ironic_pxe_append_params %}
-pxe_append_params = {{ kolla_ironic_pxe_append_params | join(',') }}
+pxe_append_params = {{ kolla_ironic_pxe_append_params | join(' ') }}
 {% endif %}
 
 {% raw %}