Updates server_group for 2.0.0

Replaces the policies argument with policy. The policies attribute is
ancient and was superceded by policy a long time ago[0][1]. Since policy and
policies differ in type, we can't simply alias these and had to drop
policies to keep the implementation small.

[0] https://docs.openstack.org/nova/latest/reference/api-microversion-history.html#id59
[1] https://docs.openstack.org/api-ref/compute/#create-server-group

Change-Id: Icd658fb179ca0a96276b033130b9dddfedd84236