[SRU] no option to override the fixed_subnet when creating a new cluster

[Felipe Reyes]

[Impact]

When a cluster template sets fixed_network and fixed_subnet and the user tries to create a new cluster using that template and decides to override the network, the fixed_subnet will inherited from the template, leaving an invalid configuration and later Neutron will refuse to allocate a port (since the subnet doesn't belong to the network). For more details see https://bugs.launchpad.net/ubuntu/+source/magnum/+bug/2038109

[Test Case]

1. Deploy an OpenStack cloud with the magnum-ui extension installed.
```
git clone https://opendev.org/openstack/charm-magnum-dashboard
cd charm-magnum-dashboard
git checkout stable/${VERSION} # ${VERSION} can be 2023.1, yoga or ussuri.
tox -e build
tox -e func-target -- ${BUNDLE} # ${BUNDLE} can be jammy-antelope, jammy-yoga or focal-ussuri
```

2. Create a new cluster template with a fixed_network test-net and fixed_subnet test-subnet
3. Create a new cluster, uncheck the option "Create new network" and pick a network different from test-net in the dropdown list.

Expected result:

The cluster gets created

Actual result:

The cluster creation fails, because the network configuration is invalid.

[ Where problems could occur ]

This is a javascript (Angular) code change, issues can be detected using the Web Developer Tools console where a javascript exeception may be raised.

Another source of problems is that this code change adds a handler when populate the list of subnets when the network is changed in the "Fixed Network" dropdown list, if there were issues the subnet list would be rendered empty.

[ Other Info ]

- This bug fix was merged during the 2024.1 (Caracal) development cycle - https://review.opendev.org/c/openstack/magnum-ui/+/898007
- The commit that fixes this issue is available since magnum-ui-14.0.0 - https://opendev.org/openstack/magnum-ui/commit/6f6c3db282fe2f0e08ad69c557eb153858b0164a
- This bug fix is not relevant for upgrades, it's purely UI fix
- This change is adding a new UI component, which on the surface may look like not suitable for a SRU, although the current UI induces users to get into a broken configuration for new clusters when overriding the cluster template's network.

[Felipe Reyes (freyes)]

https://review.opendev.org/c/openstack/magnum-ui/+/898007

[OpenStack Infra (hudson-openstack)]

This issue was fixed in the openstack/magnum-ui 14.0.0.0rc1 release candidate.

[Felipe Reyes (freyes)]

Removing tasks for the releases that are EOL.

[Felipe Reyes (freyes)]

Marking Bobcat as won't fix since it's going EOL and this patch is not needed when doing upgrades.

[Felipe Reyes (freyes)]

Zed is EOL

[Felipe Reyes (freyes)]

marking magnum-ui as fix released, since the patch landed during the Caracal devel cycle and it's part of magnum-ui-14.0.0 - https://github.com/openstack/magnum-ui/commit/6f6c3db282fe2f0e08ad69c557eb153858b0164a

[Felipe Reyes (freyes)]

marking magnum-ui (ubuntu)/devel as invalid since the fix is part of the package shipped with Noble

[James Page (james-page)]

Sponsored for focal and jammy for SRU team review.

Uploaded to antelope UCA.

[Chris Halse Rogers (raof)]

How does this interact with https://bugs.launchpad.net/ubuntu/+source/magnum/+bug/2038109 ? It looks like maybe it does? Do these need to be tied together at all?