Enable application credential support

[Dmitrii Shcherbakov]

OpenStack application credentials (as of Queens) allow users to create credentials usable via CLI or in API clients that have an ID and a secret with a subset of user roles and an optional expiration date.

https://docs.openstack.org/keystone/queens/user/application_credentials.html
https://developer.openstack.org/api-ref/identity/v3/#application-credentials

This approach is preferred when an underlying cloud is integrated with a user directory (via LDAP) and usage of usernames and passwords is undesirable.

The support for using them is not yet in upstream K8s at the time of writing:
https://github.com/kubernetes/kubernetes/blob/release-1.15/pkg/cloudprovider/providers/openstack/openstack.go#L149-L170
https://github.com/kubernetes/cloud-provider-openstack/blob/4f9c1d0ff865a163ce3efaecd1003d4a308c7d76/pkg/cloudprovider/providers/openstack/openstack.go#L172-L189

However, once it is merged, we need to enable it in the openstack-integrator charm and the charms that use it.
https://github.com/kubernetes/cloud-provider-openstack/pull/426

[Cory Johns (johnsca)]

The referenced PR is available in the cloud-provider-openstack used in cdk-addons >= 1.18, so we could add support for this as long as the channel is that or later.