Parallelism: set used cpu cores manually

[JFGHT]

What problem does this feature solve?

Dockerized services limit the amount of CPU cores that can be used and, the service itself does not report correctly how many you can use from the actual total amount.

Example: #2208

What does the proposed API look like?

Global config file has the property parallel as a boolean. It could be just a number, the number of cores that are gonna be used, being 0 as parallelism deactivated. Or just some other way that allows us to set manually the amount of cores.