Add to list of distributions that don't support newer dhcpd.conf options

Change-Id: I21cedd0877ba414769431c54825e9403e494fcee
diff --git a/templates/unbound.conf.j2 b/templates/unbound.conf.j2
index 3dbddea..a46414c 100644
--- a/templates/unbound.conf.j2
+++ b/templates/unbound.conf.j2
@@ -19,7 +19,7 @@
   # caching overrides
   # Cache SERVFAIL for less time (normally 900s)
   infra-host-ttl: 10
-{% if ansible_distribution_version != "18.04" %}
+{% if ansible_distribution_version not in ["18.04", "20.04"] %}
   # keep probing failed hosts, in case of network issues
   infra-keep-probing: yes
 {% endif %}