health-server: Using lower-case scheduling name

There is no need to get Upper case here. Pretty confusing with the rest
of the tool.
health-server: Handling no more hosts in fair scheduling

While computing the selected host with the fair affinity we shall stop
computing the list if one host doesn't have no more hosts available on
it.
health-server: Switching logging to INFO

There is no need being at DEBUG level all the time. That's pretty
floody.
health-server: Adding affinity-hosts support

It's now possible to define a list of hosts that shall be used for a
benchmark. This is very useful when running ahc in distributed mode to
selects VMs from a particular hypervisor.

Inside a job, you can use the comma-separated 'affinity-hosts' option to
select the host you want to use during this test.

The affinity-hosts is made of host's UUIDs like :
    affinity-hosts : 44454c4c-4b00-1039-8050-b9c04f573032