Do not create CAs with empty subjects

The x509_ca_subject variable shouldn't have an empty default.

playbooks/init-credentials.yml

@@ -44,4 +44,4 @@
       local_action: command openssl dhparam -out "{{ credentials_dir }}/x509/dhparam" "{{ dhparam_bits | default('2048') }}" creates="{{ credentials_dir }}/x509/dhparam"
 
     - name: Generate the X509 CA certificate
-      local_action: x509_ca ca_subject="{{ x509_ca_subject | default('') }}" ca_cert_path="{{ credentials_dir }}/x509/ca.pem" ca_key_path="{{ credentials_dir }}/x509/ca_private_key.pem"
+      local_action: x509_ca ca_subject="{{ x509_ca_subject | default('CN=Service CA') }}" ca_cert_path="{{ credentials_dir }}/x509/ca.pem" ca_key_path="{{ credentials_dir }}/x509/ca_private_key.pem"