Old server group utils

[MaximMonin]

I recently had a problem with soft-anti-affinity policy when creating a cluster.
Invalid input for field/attribute 0. Value: soft-anti-affinity. 'soft-anti-affinity' is not one of ['anti-affinity', 'affinity']
Issue described in #463

And fixing it with

[nova_client]
api_version = 2.15

helped me.

But I think the real problem is deeper.
The current magnum driver at https://github.com/vexxhost/magnum-cluster-api/blob/main/magnum_cluster_api/utils.py#L656 still uses the 2.63- server group creation notation, providing the "policies" input parameter. But with microversion 2.64+, the "policy" parameter is used. And this required a rework of the util.py.