No need for host groups, just use host aliases

hosts.ini

-[giffard]
 giffard ansible_host=giffard.streampunk.cc public_ip=49.12.77.64
-[blimp]
 blimp ansible_host=blimp.streampunk.cc public_ip=116.203.245.27
-[zeppelin]
 zeppelin ansible_host=zeppelin.streampunk.cc public_ip=95.217.20.254
 
-[hetzner:children]
+[hetzner]
 giffard
 blimp
 zeppelin
@@ -20,11 +17,11 @@ hetzner
 [etcd-master:children]
 hetzner
 
-[monitor:children]
+[monitor]
 giffard
 blimp
 
-[test-source:children]
+[test-source]
 zeppelin
 
 [all:vars]