For rightsizing ask for existing compute usage

[jolynch]

Right now most models are split into two parts:

1. Try to determine the resources you need for a desire using math on the desire (CPU, RAM, Disk, Network, etc ...). Example
2. Size and price clusters based on that particular service deployment mode (e.g. C* has to scale by factors of 2, and deploys to zones). Example

This makes sense for provisioning where we are trying to guess CPU time from e.g. payload sizes and RPS and such. For rightsizing it might makes more sense to just provide existing choices in the desire along with utilization and then the model can produce a ideal hardware for that specific requirement. Perhaps modify CapacityDesires to have an additional field called existing_deployment that takes either a Requirements or a Clusters. Maybe with the modification of instead of supplying a frequency to requriements, have a hardware shape/count (the cpu_count would be cpu * utilization for example).

Then models can short circuit the requirements generation or at least use the provided numbers as good defaults. RAM is the only one that seems tricky to me that might require merging.