Implement runners

[dbarrosop]

The idea is to backport the runners plugin from gornir to nornir. A runner is a function that implements how to run the tasks over the given hosts. As an example, nornir already ships _nornir_serial (runs only one host at a time) and _nornir_parallel (runs #num_workers threads), the idea is to decouple those from the core and implement them as "plugins" instead so anyone can write their own runners with their own strategy.