Collect "nomadic" executors in a list.

Nomadic executors are ones that aren't successfully, locally scheduled.

If we're going to schedule all components, we'll need this complete list.
Pass executors that weren't scheduled locally to complete().

Nothing is done with them yet.
Prove components that fail local scheduling are passed to complete().
Prove indifferent executors are also passed to complete().
Don't veto system components outright.
Include alien executors with the nomads.
Add a failing test for choosing extra hosts.
Initial stab at choosing more hosts to satisfy numWorkers.
Make the test pass.

Turns out the test's premise was wrong; to exercise the new functionality, we have to request more workers than the assigned hosts collectively have, after considering the remaining executors.