health-server: Fixing bit-logic

The CPU flag was never been set in the host_state variable and never
cleared....
health-server: Adding range support for required-hosts

If you want to run the same load on a set of servers, you can use the
following syntax to define your range.

required-host : <min>-<max>

By default, the range will be used by incrementing by 1. If you need to
increment by any other value, the step-host parameter is used :

step-host : <value>

An example to test from 1 to 100 VMs and testing by increment of 10 VMs:

name: cpu_load
required-hosts: 100
runtime: 1
cpu:
    required-hosts: 1-100
    step-host: 10
    runtime: 10
    cores : 2