Balloons: consider allowing multi-instance reserved balloons

[askervin]

Use case: In a multi-socket system, I want one reserved balloon instance on every socket so that kube-system and other reserved containers are spread evenly on every socket.

Currently it's possible to create a custom balloon type or modify the default balloon type to behave like this, but if it captures kube-system containers, then the reserved balloon will stay empty and yet it is forced to be at least of size 1 CPU.

An alternative solution to avoid wasting one CPU would be to allow the reserved balloon can be of size 0 CPUs.

Consider which one would be better, or should we have both solutions available.