Add support for `docker_volume_{size,type}`

[mnaser]

At the moment, we cannot use this feature due to the fact that CAPO does not support mounting multiple volumes:

https://github.com/kubernetes-sigs/cluster-api-provider-openstack/issues/1286

Once that's in place, we can use the following labels to create volumes:

• etcd_volume_size
• etcd_volume_type
• docker_volume_size
• docker_volume_type

We can use the following functions to determine the volume type (if size is set):

https://github.com/openstack/magnum/blob/16bdedcf2fe6986c995bd415f4e3c70dac914ada/magnum/common/cinder.py#L25-L37

And we can get the sizes from labels/API objects.

[mnaser]

For what it's worth, this isn't that important, since most users will find https://github.com/vexxhost/magnum-cluster-api/issues/5 more important since that gives them BFV instances.