MINOR: cpu-topo: add a CPU policy setting to the global section

MINOR: cpu-topo: add a CPU policy setting to the global section

We'll need to let the user decide what's best for their workload, and in
order to do this we'll have to provide tunable options. For that, we're
introducing struct ha_cpu_policy which contains a name, a description
and a function pointer. The purpose will be to use that function pointer
to choose the best CPUs to use and now to set the number of threads and
thread-groups, that will be called during the thread setup phase. The
only supported policy for now is "none" which doesn't set/touch anything
(i.e. all available CPUs are used).