New -single-worker option used to use 1 worker by system

[sergiocazzolato]

The number of workers can be set as part of the spread.yaml file and also by using the -single-worker option, which indicates that just 1 worker is going to be used for the run.

[mvo5]

This is interesting - we may instead use a way to specify "limit-to-a-single-one" to avoid side-effects like creating thousands of machines (e.g. --single-worker).

[sergiocazzolato]

@mvo @niemeyer I updated the change to allow the new option -single-worker as it was requested on Frankfurt