add support for notify_on_state_change in Nova Cloud Controller charm for Fleio compatibility

[Fulga Fabian]

Currently, the Nova Cloud Controller charm does not support configuring the notify_on_state_change option, which is required for Fleio integration as described in their documentation: https://fleio.com/docs/2024.10/configuring/enable-openstack-notifications.html#nova-notifications. This option allows Nova to send notifications when there are changes in the VM state or task state, which is crucial for services that rely on real-time updates from Nova.
The required configuration is as follows:

[DEFAULT]
notify_on_state_change = vm_and_task_state

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/c/openstack/charm-nova-cloud-controller/+/933007

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-nova-cloud-controller/+/933007
Committed: https://opendev.org/openstack/charm-nova-cloud-controller/commit/e1b1da051516d1c49b07ff4ec8dff4958a4e779d
Submitter: "Zuul (22348)"
Branch: master

commit e1b1da051516d1c49b07ff4ec8dff4958a4e779d
Author: Fabian Fulga <email address hidden>
Date: Tue Oct 22 15:47:22 2024 +0300

    Add support for `notify_on_state_change` in Nova Cloud Controller charm

    This change introduces two new configuration options to the Nova Cloud Controller charm:

    - `enable-notify`: Allows users to enable the `notify_on_state_change` setting in the Nova Cloud Controller. This controls whether notifications should be sent for changes in instance states. If `enable-notify` is set to `True`, users can configure the `notify-on-state-change` option to specify the types of state changes to notify on.

    - `notify-on-state-change`: Specifies when to send `compute.instance.update` notifications on instance state changes. The possible values are:
      - `None`: Disables notifications.
      - `vm_state`: Sends notifications on changes to the VM state.
      - `vm_and_task_state`: Sends notifications on changes to both VM and task states.

    If `enable-notify` is set to `True`, the following configuration is added to `nova.conf`:

        [DEFAULT]
        notify_on_state_change = {{ notify_on_state_change }}

    By default, `enable-notify` is set to `False`, keeping the existing behavior unchanged unless explicitly configured. This ensures that no notifications are sent unless the user opts in, providing backward compatibility while offering the flexibility to enable state change notifications when needed.

    Closes-Bug: #2085354
    Change-Id: I2f8926a89c89b03e81d05d36dd65579eb8bec16c