Add nova-status ready-for-upgrade command

This has been discussed a few different times in Ocata when talking about how to enforce and/or perform a readiness check when upgrading nova from newton to ocata because the placement service and cells v2 are going to be required in Ocata.

There are notes in the etherpad here:

https://etherpad.openstack.org/p/nova-ocata-ready-for-upgrade

We already have an API DB migration blocker for cells v2 in the o-2 milestone, but we need something for the placement service since that needs to check the cell (nova) and API DB to see that (1) the placement service is running and (2) there are resource providers getting populated from the compute nodes, which was available but optional in Newton, but is required to use the scheduler in Ocata due to:

https://specs.openstack.org/openstack/nova-specs/specs/ocata/approved/resource-providers-scheduler-db-filters.html

So we're going to add a new 'nova-status' CLI entrypoint, similar to nova-manage but without the old cruft in the nova-manage command, so a new cleaner argparse handler and structure. This will initially be developed within nova but might be split out later. The idea is to run this after installing the Ocata package code and running the DB migrations, but before restarting the services to use the new code.