[objectstorage] Gophercloud accepts container names with a slash character

As per the [OpenStack object-storage docs](https://docs.openstack.org/api-ref/object-store/#create-container), container names must not contain a slash (`/`).

By accepting them, some functions change their meaning. For example, by passing a container name with a slash to `containers.Create`, it is possible to actually create an object (if the container exists).