Don't save registry logs if registry isn't running

Now that we use the buildset-registry job in all docker builds
(whether they run the registry internally or not), in the post
playbook we need to check whether the registry was actually
started in the pre playbook before we attempt to save its logs.

Change-Id: Ifa925faf7bfdc83238677261726aa96461faddb0

playbooks/buildset-registry/post.yaml

@@ -4,12 +4,18 @@
 
 - hosts: all
   tasks:
+    - name: Check if buildset registry is running
+      command: "docker ps"
+      register: docker_ps
+
     - name: Create container log dir
+      when: "'buildset_registry' in docker_ps.stdout"
       file:
         path: "{{ ansible_user_dir }}/zuul-output/logs/docker"
         state: directory
 
     - name: Save registry container logs
+      when: "'buildset_registry' in docker_ps.stdout"
       loop:
         - buildset_proxy
         - buildset_registry