Implement Zero Downtime Password Rotation (ZDPR) support for IronicNeutronAgent using Keystone Application Credentials

Description:

Implement Application Credential authentication support for the IronicNeutronAgent service to enable Zero Downtime Password Rotation (ZDPR).

Acceptance Criteria:

• IronicNeutronAgent controller watches the AppCred Secret (ac-ironic-secret)
• Configuration templates render auth_type=v3applicationcredential with AC_ID and AC_SECRET when AppCred Secret is available
• All auth sections in IronicNeutronAgent config support AppCred - sections:
  • [keystone_authtoken]
  • [service_catalog]
  • [ironic]
• IronicNeutronAgent deployment triggers a rolling update when the AppCred Secret changes
• Falls back cleanly to password authentication (auth_type=password) when AppCred Secret is missing or incomplete
• All configuration changes are backwards compatible with password-based auth
• Redeployment with new credentials during the grace period works without service disruption

Technical Context:

• Controller: controllers/ironicneutronagent_controller.go
• Config template: templates/ironicneutronagent/config/01-ironic_neutron_agent.conf
• Current auth method: password-based