Modernize the os-hypervisors API

The 'os-hypervisors' API is around since the early days of nova. Back in those halcyon days, it provided a nice way to get a quick summary of the resource usage of individual compute nodes in your deployment. Today, however, it's a shell of itself. The more detailed information it returns is hypervisor-specific and frequently wrong, especially with advanced features like CPU pinning or file-based memory. With the elevation of placement to its rightful place as lord of (almost) all things resource'y, the 'os-hypervisor' API needs to slim down significantly.