knotd: do not try to load zone file of slave zones

roles/knotd/templates/knot.conf.j2

@@ -104,10 +104,7 @@ template:
   - id: default
     storage: "{{ knotd__zones_dir }}"
     file: "%s.zone"
-    semantic-checks: {{ knotd__semantic_checks
-                        | ternary("on", "off") }}
     zonefile-sync: -1
-    zonefile-load: difference-no-serial
     journal-content: changes
     journal-content: all
     serial-policy: {{ knotd__serial_policy }}
@@ -128,6 +125,11 @@ zone:
 {% endif %}
 {% if zone.master is defined %}
     master: {{ zone.master }}
+    zonefile-load: none
+{% else %}
+    zonefile-load: difference-no-serial
+    semantic-checks: {{ knotd__semantic_checks
+                        | ternary("on", "off") }}
 {% endif %}
 {% if zone.dnssec_policy is defined %}
     dnssec-policy: {{ zone.dnssec_policy }}