crimson: introduce crimson_seastar_cpu_cores

crimson: introduce crimson_seastar_cpu_cores

seastar::smp::count is deduced from the cpuset provided.

from seastar/src/core/reactor:
```
   if (smp_opts.smp) {
        smp::count = smp_opts.smp.get_value();
    } else {
        smp::count = cpu_set.size();
    }
```

Co-authored-by: Yingxin Cheng <yingxin.cheng@intel.com>
Signed-off-by: Matan Breizman <mbreizma@redhat.com>