CAPO - Additional data volumes for machines

This task in CAPO will add the ability for a machine to have additional data volumes attached, e.g. for etcd.

This can be volumes or ephemeral disks.

See https://github.com/kubernetes-sigs/cluster-api-provider-openstack/issues/1286 for details and upstream discussion.

Matt Pryor from StackHPC started the work but he needs some help so we'll collaborate on this one.

This task will probably be delivered into 2 PRs:

• https://github.com/kubernetes-sigs/cluster-api-provider-openstack/pull/1668
• another PR to support local disks for ephemeral storage and not just cinder volumes