health: Reporting affinity properly

Metrics shall have the VM/host affinity reported in the affinity
variable.

In the network mode, let's it this way.
health: Return if no more hosts

If no more hosts to find, let's return explicaly instead of breaking.
There is no more need to search for something else.
health: Cancel test if nb_host modulo arity is not 0

Sanity check to insure we'll never run with a nb_host which is not
modulo the arity we do expect.

If so that would mean that the load isn't fair.