Problem: A user with a reservation at one cluster, was trying to start
an experiment at a different cluster. But after selecting the other
cluster and clicking next, that selection was being reset to the cluster
with the reservation, as part of our overly helpful cluster picker code,
which is fired off in a callback from Jacks.

Question: Why is that happening when clicking next, to go from the
Finalize step to the Schedule step, thus silently resetting the user's
choice.

Reason: There is a call to SwitchJacks("small") to make sure that the
small version of the topo was visible, in case the user goes back to the
finalize step. Not sure why ... but in any event, every call to
SwitchJacks(large|small) results in Jacks firing that callback, and
resetting the user's cluster choice.

Solution: Null out the callback when calling SwitchJacks().

There is probably a better solution to this, perhaps as part of a code
reorg on the entire instantiate page, which has grown many appendages
over time.