I'm not sur that this is the way to go. 
We already have the notion of the special target "all servers", just that for now, it's a system target. 
But if you add that LDIF in the root category, you're going to see it:
```
  dn: ruleTarget=special:all,groupCategoryId=GroupRoot,ou=Rudder,cn=rudder-configuration
  objectClass: specialRuleTarget
  objectClass: top
  cn: All servers
  ruleTarget: special:all
  description: A special target which matches absolutly all known nodes, including policy servers
  isEnabled: TRUE
  isSystem: FALSE
```
But kind of target is supported in all backend part of Rudder, but not completely in the front end (in Rules, for example, you can not select that target). 

So, in the one hand, your solution is a better "I made a first example for you", as it IS a normal dynamic group, deletable and all, and moreover, the patch is really short. 

On the other hand, its a pitty (for perf and consistance) to not use the target:all for that case. But using it means adapting the UI in a lot of place to take account of that. For example, it could mean addind the possibility to create "special:all" group, even if it's likelly to never happen. 

So... I ask for some other RT :)