[manila-csi-plugin] Support for `cloud-config` secret

/kind feature

**What happened**: All the other OpenStack-related plugins support a `cloud-config` secret that contains credentials for authentication with the keystone. Conversely, Manila CSI Plugin wants its own format for auth secrets, making it difficult to integrate it with existing Kubernetes-on-Openstack environments (e.g., the Charmed Kubernetes Distribution.

**What you expected to happen**: it would be easier to mount the same `cloud-config` secret in all the OpenStack plugins ecosystem, instead of having a different integration path just for Manila. Is it something feasible?