For quite some time, we relate usage of --by-service flag for
nova-manage cell_v2 discover_hosts command to the used nova_virt_type.
However, we run db_post_setup tasks only once and delegating to the
conductor host. With latest changes to the logic, when this task in
included from the playbook level it makes even less sense, since
definition of nova_virt_type for conductor is weird and wrong.
Instead, we attempt to detect if ironic is in use by checking hostvars
of all compute nodes for that. It will include host_vars, group_vars,
all sort of extra variables, etc.
Thus, ironic hosts should be better discovered now with nova-manage
command.
Related-Bug: #2034583
Change-Id: I3deea859a4017ff96919290ba50cb375c0f960ea