Better CPU platform defaults #9079
Description
#8728 added CPU platforms to instances and the API to set them, but a kind of underwhelming approach to picking the default CPU platform for an instance if none is specified: "the most general platform for whatever hardware the instance gets placed on". This is fine for existing Gimlet-based racks, because the most general platform on a Gimlet is also the only CPU platform a Gimlet can run, which is amd_milan.
On racks with Cosmos in them, instances will default to a CPU of amd_milan even if the only available hardware is Cosmo which can run amd_turin. Even if the rack only has Cosmo! Even if the operator intends to only ever install Cosmo (or later)!
I suspect this will fit reasonably alongside #9078, probably around Silo quotas, but I haven't thought very deeply about it yet.