No way to set allocate-public-ip=true globally on OpenStack substrates

Previously Juju had a model config option as use-floating-ip=true which can be used with model-defaults to apply the config globally to new models. However, the model config for use-floating-ip is deprecated by now and replaced by a constraint as allocate-public-ip=true instead.

We can set the constraint with set-model-constraints but it's a per model constraint not like a global default for a substrate with model-defaults as previous.

[previous behavior]

juju model-defaults openstack/regionOne \
    network=MyPrivateNetwork \
    use-floating-ip=true

juju add-model my-new-model1

juju add-model my-new-model2

[current behavior]

juju model-defaults openstack/regionOne \
    network=MyPrivateNetwork

juju add-model my-new-model1
juju set-model-constraints allocate-public-ip=true

juju add-model my-new-model2
juju set-model-constraints allocate-public-ip=true