[loadgen] ability to include previously stopped workspaces

[kylos101]

Is your feature request related to a problem? Please describe

Right now we start new workspaces

Describe the behaviour you'd like

It would be great to start existing workspaces, so we can test content restore / snapshot restore.

Describe alternatives you've considered

n/a

Additional context

Right now, we know that snapshot restore is a constant (roughly 30s).

[kylos101]

Testing

[github-activity]

Testing (non-gitpodder-account)