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