SCALE_IN Action Bug - Inconsistent Node Deletion Behavior

[Nguyen Ngoc Hieu]

Behavior: With the action SCALE_IN, it consistently removes only one node from the load balancer (LB), even when the count of inputs is greater than 1 (in cases where there is no deletion data).

Reproduce this bug:
- Create a cluster with more than 1 node.
- Attach lb_policy.
- Perform SCALE_IN with a count number greater than 1.
=> Only one node is deleted and removed from the LB.

Root cause: pre_op -> _get_delete_candidates
https://github.com/openstack/senlin/blob/541498724bc0c1cf730d9f31c2ca7cf218d07bc3/senlin/policies/lb_policy.py#L528
https://github.com/openstack/senlin/blob/541498724bc0c1cf730d9f31c2ca7cf218d07bc3/senlin/policies/lb_policy.py#L544-L550

I've previously reported this [https://bugs.launchpad.net/senlin/+bug/1966847], but it seems the description is not clear.

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/senlin/+/904800

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/senlin/+/904800
Committed: https://opendev.org/openstack/senlin/commit/2b6693576355b673dcc4e3e53f1291204412c512
Submitter: "Zuul (22348)"
Branch: master

commit 2b6693576355b673dcc4e3e53f1291204412c512
Author: Nguyen Ngoc Hieu <email address hidden>
Date: Fri Jan 5 02:26:04 2024 +0700

    fix: SCALE_IN action not honoring specified node count

    The SCALE_IN action in the lb_policy module was removing only
    one node from the load balancer, regardless of the specified
    count when the count was greater than 1. This was due to a
    hard-coded count of 1 in the _get_delete_candidates function.

    The root cause has been addressed by updating the
    _get_delete_candidates function to dynamically handle the count
    for SCALE_IN. The count is now retrieved from the action inputs,
    and if not specified or invalid, it defaults to 1.

    Closes-Bug: #2048099
    Change-Id: I9ff12dec26f4c558ef5ef84de87668e9668cec80