Cinder backends should be able to have same name

[Lukas Sabla]

**Bug Report**

What happened:

Template cinder.conf.j2 generates same cinder backend [name] and volume_backend_name.

What you expected to happen:

According to the official documentation (https://docs.openstack.org/cinder/latest/admin/multi-backend.html) we should be able to set same volume_backend_name for multiple cinder backends. We use single volume_backend_name for multiple backends in produciton, with scheduler filtering by availabity zones.

Template file should use variable volume_backend_name if defined in cinder_ceph_backends or use name if volume_backend_name not defined.

**Environment**:
* Kolla-Ansible version: c5f3f23e05a901bf2663184f13c137e46d71f507