health: Dump early metrics in result dir

When something is getting wrong during a benchmark, its pretty
frustrating having no metrics file as their are only dumped at the end.

This patch generates a premilinary metrics file to provide the context
_before_ the benchmark which will be overwritten at the end.
health: Fixing network host computation

The computation of the number of available host was considering the
number of hypervisors and not the number of hosts.

This did prevent the benchmark to run.