New tbswap mode 'update'. (a.k.a. 'reswap')
Re-assigns experiment, fixing already assigned nodes in place;
tries not to reboot nodes. Doesn't clear port counters,
restart event system, etc.

A few more things remain to be considered for 'general' use
adding new nodes to experiments and modifying topologies,
but for replacing failed nodes in experiments
or removing virt_nodes from experiments, should work fine.