Add spot vs on-demand to API configuration

[deliahu]

Description

Add a field to API configuration to specify whether to run the replicas for an API on on-demand instances or spot instances.

Motivation

Designate high-priority APIs to run on on-demand instances, and low-priority APIs to run on spot instances.

Questions

• How does this relate to the other spot settings (e.g. on_demand_base_capacity and on_demand_percentage_above_base_capacity)? Will it be a choice between the two existing ASGs (a single spot ASG and a single on-demand ASG), or will these settings be separately configurable per API, in which case each API would get it's own ASG?