health-server: Using global variable for hosts

When using hosts, let's insure we do use the global value.
health: Cancel test if not enough hypervisors

If user setup a set of hypervisors to use for the benchmark, it's
important having all of them before starting unless you could in this
case :

- Select 2 HV in the config file
- Only 1 HV is used as you have a typo in the config file
- Run the test

All the tests ran on the same HV while you expected 2.