Use the existing nova-compute cellv2 discovery logic for nova-ironic too, now
that we have the --by-service flag.
The nova_api_discover_hosts.sh script will now wait (up to 10 minutes) for all
nova-compute and nova-ironic services to register, then run host discovery
with --by-service to create host mappings for all services. We no longer need
ironic nodes to be deployed on the nova-ironic services for discovery to work.
We also no longer need to enable the priodic job.
Related nova change Ie9f064cb9caf6dcba2414acb24d12b825df45fab
Related-Bug: #1755602
Change-Id: I723237ae7285f3babd6eceb1ce7da4e2734d1e4f
9d61779a23