Add a default for kolla_build_args

It is not an official Kayobe variable, so doesn't have a default.

Author: markgoddard

etc/kayobe/kolla/kolla-build.conf

@@ -8,7 +8,7 @@ base_tag = jammy-20231004
 {% elif kolla_base_distro == 'rocky' %}
 base_tag = 9.{{ stackhpc_pulp_repo_rocky_9_minor_version }}
 {% endif %}
-build_args = {{ kolla_build_args.items() | map('join', ':') | join(',') }}
+build_args = {{ (kolla_build_args | default({})).items() | map('join', ':') | join(',') }}
 
 [openstack-base]
 type = git