Add option to assign a slave by name or groups of slaves.
"Restrict where this project can be run" seems to be a standard option of normal "free-style" jobs

Alternatively, perhaps better:

• A Flow is really only part of the job scheduling which is normally done by the master. So it should run always on the master.
• Also the currently running Flow(s) should be shown on the LHS but not associated with any particular slave. Rather in their own list.