feat: Support Cinder multibackend configs.

[dulek]

Is this a bug report or feature request? (choose one): feature

A single cinder-volume service is able to handle multiple volume backends. To make sure openstack-helm can be used with any Cinder-supported storage hardware/driver we should extend Cinder's charm to handle configuring any backend.

[v1k0d3n]

this the same as #321? @wilkers-steve has a PR in currently that will bring LVM (#300), and this will ultimately take advantage of the work that @alanmeadows has done in #262, which I mentioned in your recent Cinder PR.

[dulek]

Thanks, I'll make sure this have the shape to fit @wilkers-steve and @alanmeadows efforts.