Define how many parallel processes gets used

[MindTooth]

Is your feature request related to a problem? Please describe

Setting ControlMaster throws an error when mani creates to many processes:

mux_client_request_session: session request failed: Session open refused by peer
ControlSocket /tmp/git@<url>:22 already exists, disabling multiplexing

Being able to limit somehow would be nice.

Describe the solution you'd like

Add the possibility to set how many parallel processes created when invoking --parallel.

Additional context

[alajmo]

Fixed with 0.30.0. You can set forks when running tasks (--parallel --forks=8) or set it via a spec:

specs:
  default:
    parallel: true
    forks: 8

It's set to 4 by default.