inventory: add neutron 2025.2 service groups

[ideaship]

What

Add three kolla inventory groups to inventory/51-kolla, all mapping to control (mirroring neutron-server):

• [neutron-rpc-server:children]
• [neutron-periodic-worker:children]
• [neutron-ovn-maintenance-worker:children]

Why

kolla-ansible stable/2025.2 split three new, default-enabled services out of neutron-server, each carrying host_in_groups: "{{ inventory_hostname in groups['<name>'] }}". Building the haproxy-config with_dict loop templates the whole neutron_services dict, so every entry's host_in_groups is evaluated; a lookup on a group absent from this inventory aborts templating with 'dict object' has no attribute 'neutron-rpc-server' (the "dict object" is the groups var, not neutron_services).

This inventory shipped neutron-server but never gained the three new groups, so testbed-upgrade-stable-next-ubuntu-24.04 fails during "Run upgrade" once the 2025.2 neutron role is in play. The failure surfaces on neutron-rpc-server first only because templating stops at the first missing key — the other two groups are the same gap and would surface next.

stable/2025.2 ansible/inventory/multinode places all three under control, which this change matches.

Context

This is one wall in a 2025.2-upgrade "gauntlet" of new-service wiring deficits: each cleared blocker exposes the next. The immediately preceding wall was the keystone-httpd image ('docker_image_url' is undefined). Image overrides for these neutron services already exist in osism/defaults; this closes the remaining inventory-group side.

Timeline (observed)

Date	Failure point
06-19 / 06-20	keystone-httpd image (docker_image_url undefined) — before neutron
06-21	this gap (neutron-rpc-server group) — first reached
06-22 / 06-23	masked by earlier pre-neutron failures (SSH timeout, bootstrap)
06-24	this gap again

Caveat

Won't go quiet until merged and the nightly manager picks up the new osism.generics tag via gilt. Clearing neutron may expose whatever is immediately downstream — but the static wiring gap is closed here.