No way to specify a common volume_backend_name across multiple backends

[Nobuto Murata]

OpenStack actually allows setting the same volume_backend_name across multiple storage backends.
https://docs.openstack.org/cinder/ussuri/admin/blockstorage-multi-backend.html#enable-multiple-storage-back-ends

However, the charm takes the approach to set it from the application name in Juju. So at this moment, the same name for volume_backend_name cannot be set since Juju doesn't allow the same application in the model. Would be nice to pass an arbitrary volume_backend_name through a charm config.

[cinder-ceph-c1]
volume_backend_name = cinder-ceph-c1
volume_driver = cinder.volume.drivers.rbd.RBDDriver
rbd_pool = cinder-ceph-c1
rbd_user = cinder-ceph-c1
rbd_secret_uuid = b0ff5b30-9b5f-48b0-8827-750924276a19
rbd_ceph_conf = /var/lib/charm/cinder-ceph-c1/ceph.conf
report_discard_supported = True
rbd_exclusive_cinder_pool = True
rbd_flatten_volume_from_snapshot = False

[cinder-ceph-c2]
volume_backend_name = cinder-ceph-c2
volume_driver = cinder.volume.drivers.rbd.RBDDriver
rbd_pool = cinder-ceph-c2
rbd_user = cinder-ceph-c2
rbd_secret_uuid = 0afd2211-14ae-4d65-99c9-659a3f0f7377
rbd_ceph_conf = /var/lib/charm/cinder-ceph-c2/ceph.conf
report_discard_supported = True
rbd_exclusive_cinder_pool = True
rbd_flatten_volume_from_snapshot = False

[OpenStack Infra (hudson-openstack)]

Fix proposed to branch: master
Review: https://review.opendev.org/737506

[OpenStack Infra (hudson-openstack)]

Reviewed: https://review.opendev.org/c/openstack/charm-cinder-ceph/+/737506
Committed: https://opendev.org/openstack/charm-cinder-ceph/commit/d9873fd22cf1d5081344bde58b324cfb84a5c693
Submitter: "Zuul (22348)"
Branch: master

commit d9873fd22cf1d5081344bde58b324cfb84a5c693
Author: Nobuto Murata <email address hidden>
Date: Tue Mar 30 15:08:59 2021 +0900

    Add support of a common volume_backend_name

    Previously, we didn't have a control over volume_backend_name other than
    the default app name in the Juju model. A common backend name to
    multiple backends with the same character is useful because those can be
    treated as a single virtual backend associated with a single volume
    type.

    Change-Id: I4b57f7979837d21a1b116007f3da707ee154792b
    Closes-Bug: #1884511