Wait for all compute services before cell discovery

There is a race condition during nova deploy since we wait for at least
one compute service to register itself before performing cells v2 host
discovery.  It's quite possible that other compute nodes will not yet
have registered and will therefore not be discovered. This leaves them
not mapped into a cell, and results in the following error if the
scheduler picks one when booting an instance:

Host 'xyz' is not mapped to any cell

The problem has been exacerbated by merging a fix [1][2] for a nova race
condition, which disabled the dynamic periodic discovery mechanism in
the nova scheduler.

This change fixes the issue by waiting for all expected compute services
to register themselves before performing host discovery. This includes
both virtualised compute services and bare metal compute services.

[1] https://bugs.launchpad.net/kolla-ansible/+bug/1832987
[2] https://review.opendev.org/665554

Change-Id: I2915e2610e5c0b8d67412e7ec77f7575b8fe9921
Closes-Bug: #1835002