use-buildset-registry: do not update ca when not necessary

This change skip the update ca task when the certificate
is not modified.

Change-Id: I77d6d8771197d027774223ff5c81ab7d737672fb

roles/use-buildset-registry/tasks/main.yaml

@@ -37,9 +37,11 @@
   copy:
     content: "{{ buildset_registry.cert }}"
     dest: "{{ ca_dir }}/buildset-registry.crt"
+  register: _tls_ca
 - name: Update CA certs
   command: "{{ ca_command }}"
   become: true
+  when: _tls_ca is changed
 
 # Update daemon config
 - name: Check if docker daemon configuration exists