[DNM] [update] fix: stop control plane check during split variant post-services tests

[Valkyrie00]

Context

This is a targeted fix to unblock the job which is currently failing due to a race condition between the continuous control plane check and tobiko tests.

A more comprehensive solution, such as adding a pause/resume mechanism to the control plane check scripts or reworking how background tests interact with the split update variant, could be evaluated as a follow-up.

Problem

Some jobs fails because tobiko tests encounter VMs in BUILD state during the post-services test phase.

The root cause is a race condition: the continuous control plane check (cifmw_update_control_plane_check) runs workload_launch.sh in an infinite loop, creating and tearing down VMs every few seconds.

In the split variant, tests run between the services update and system update phases while this loop is still active.
Tobiko lists all servers, finds a VM mid-creation, and fails:
novaclient.exceptions.Conflict: Cannot 'reboot' instance ... while it is in vm_state building (HTTP 409)

Proposed solution

Stop the control plane check before running the post-services tests in update_variant_split.yml, then restart it so it monitors the system update phase as well.

The ping test is not stopped because it only runs ping against an existing VM and does not create any new OpenStack resources.

Each control plane check run produces PID-scoped log files (control-plane-test-<PID>.log), so the stop/restart cycle creates two separate runs with independent validation, no log collision.

Closes: OSPCIX-1381
Assisted-By: Cursor

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