Support activation with 2/3 read/write downstairs present.

[leftwo]

In the event that an instance is started and only 2 of the 3 downstairs are available, we should support activation with a way to allow the third downstairs to join at a later date.

See also discussion around solutions in oxidecomputer/omicron#7826
As well as RFD 564
PR for that here

[leftwo]

#1661 (comment)