[CI] Fix periodic publish of centos8 images

kolla-publish-centos8 jobs (non-stream8) have been failing on
stable/victoria with the following error: 'base_tag' is undefined. This
is because the base_tag variable is only defined for centos8s jobs in
.zuul.d/centos.yaml.

Fix by checking if base_tag is defined.

Change-Id: Iad118c9885b2134a7d071a6629a2f3e3ae3d101e

Author: priteau

tests/playbooks/run.yml

@@ -4,7 +4,7 @@
     - ../vars/zuul.yml
   vars:
     arch_suffix: "{{ '-aarch64' if ansible_architecture == 'aarch64' else '' }}"
-    tag_suffix: "{{ '-centos8s' if ansible_distribution | lower == 'centos' and base_tag == 'stream8' else '' }}"
+    tag_suffix: "{{ '-centos8s' if ansible_distribution | lower == 'centos' and base_tag is defined and base_tag == 'stream8' else '' }}"
     kolla_build_config:
       DEFAULT:
         debug: true