[cinder-csi-plugin] Rationalise all things topology

[stephenfin]

What this PR does / why we need it:

This is a patch that came out of my work on https://github.com/kubernetes/cloud-provider-openstack/issues/2861. Currently, generation of the topology request for the Cinder Volume is logically separated from the generation of the topology request for the CSI Volume. This makes things harder to understand. Move some things around to resolve this and make everything a little easier to grok. This is kept separate from https://github.com/kubernetes/cloud-provider-openstack/pull/2862 since I don't expect we'll want to backport this.

Which issue this PR fixes(if applicable):

n/a

Special notes for reviewers:

I have a decoder of the logic embedded in the commit message. Hopefully this helps with review, but please don't assume the decoder itself is correct! :sweat_smile: I have also added additional unit tests that should capture any changes in logic.

Release note:

NONE