If compute nodes are deployed without deploying/updating the controllers then
the computes will not have cellv2 mappings as this is run in the controller
deploy steps (nova-api).
This can happen if the controller nodes are blacklisted during a compute scale
out. It's also likely to be an issue going forward if the deployment is staged
(e.g split control plane).
This change moves the cell_v2 discovery logic to the nova-compute/nova-ironic
deploy step.
Closes-bug: 1786961
Change-Id: I12a02f636f31985bc1b71bff5b744d346286a95f