Fix default(omit) placeholder leak in install-config template

[tusharjadhav3302]

Why

Commit 09f3777 (merged via PR #16) introduced | default(omit) in vars: passed to ansible.builtin.template for IPI install-config generation. However, default(omit) only works in Ansible module parameters — when used in template vars, Ansible does not intercept the omit sentinel and passes the raw __omit_place_holder__... string through to Jinja2.

Any job that does not define ocp_deployment_topology.machines_subnet (e.g. osp_verification.yaml) hits this because:

1. machines_subnet_id is never set
2. default(omit) leaks the sentinel string into the template variable
3. Jinja2's is defined test returns true (the variable is defined — it just holds garbage)
4. The template renders machinesSubnet: __omit_place_holder__...

This causes openshift-install create cluster to fail with:
platform.openstack.controlPlanePort.fixedIPs[0].subnet.id: Invalid value: "omit_place_holder...": invalid subnet ID: must be a UUIDv4

The same class of bug affects installcfg_additional_trust_bundle, which would leak the sentinel into the additionalTrustBundle block when no CA cert file exists.

What

ipi_install_config.yml — template task vars:

Variable	Before	After
installcfg_machines_subnet	default(omit)	default('')
installcfg_additional_trust_bundle	default(omit)	default([])

install-config-ipi.yaml.j2 — Jinja2 conditionals:

Condition	Before	After
Render machinesSubnet	installcfg_machines_subnet is defined	installcfg_machines_subnet (truthiness)
Render externalDNS	installcfg_machines_subnet is not defined	not installcfg_machines_subnet
Render additionalTrustBundle	installcfg_additional_trust_bundle is defined	installcfg_additional_trust_bundle (truthiness)

How

Instead of relying on omit (which is intercepted only at the module-parameter level), we default to the type-appropriate empty value ('' for strings, [] for lists). The Jinja2 template then uses truthiness checks instead of is defined — an empty string and an empty list are both falsy in Jinja2, so the conditional blocks are correctly skipped when the upstream variable is absent.

Validation

• ansible-lint: Passed with 0 failures, 0 warnings (production profile)

• Scenario 1 — machines_subnet_id undefined (simulates osp_verification.yaml):

  • machinesSubnet line correctly absent from rendered output
  • No __omit_place_holder__ leak
  • externalDNS correctly present
  • additionalTrustBundle correctly absent

• Scenario 2 — machines_subnet_id = valid UUID:

  • machinesSubnet: a1b2c3d4-e5f6-7890-abcd-ef1234567890 correctly rendered
  • externalDNS correctly absent (mutually exclusive with machinesSubnet)
  • additionalTrustBundle correctly rendered with cert lines

References

• JIRA: OCPBUGS-95044
• Tracker: OSPRH-6485
• Regressing commit: 09f3777 (PR Add telco NFV day2ops procedures and IPI configuration for shiftstack #16)