Add the gnocchi_backend_storage configuration for external ceph docs

when use external ceph, kolla need set ceph to "no",this way by default, the
gnocchi_backend_storage will be file, refer to [0]. we should set
it to ceph, this patch to explain in docs.

[0] https://github.com/openstack/kolla-ansible/blob/master/etc/kolla/globals.yml#L302

Change-Id: If35082531e9de4524299fe4fc7e97bec5a6c2c79