health-server: Improve step counting

The existing code was weak as :
- it didn't insure that max_hosts was part of the serie
- we were never of a modulo of step-hosts

For a serie with min=1, max=10, step=2 we had :
1, 3, 5, 7, 9

Now we have:
1, 2, 4, 6, 8, 10

It's much more logical counting based on the modulo.