Node uptime weigher

This blueprint proposes the addition of a new scheduler weigher in OpenStack Nova that sorts compute nodes based on their uptime. This will help distribute workloads in fast-moving environments where hardware is regularly added and removed. It should help to reduce the need for live-migrations from older nodes when they are about to be retired.

Getting uptime would need a schema update and support from the compute driver to retrieve the information.

Since the uptime of a hypervisor would be driver dependent, a lower-effort version of the weigher could be implemented using the creation date of the compute service which is already known to the scheduler. However it would make the weigher less useful since a compute service can be recreated without touching the running vms or rebooting the hypervisor.