Scoped Policies for Server Groups

 Server groups are currently used to ensure that instances in the groups are either grouped together on the same host (affinity) or spread across different hosts (anti-affinity). This affinity/anti-affinity concept could be broadly useful for a number of use cases, but many of these use cases would require (anti-)affinity with a scope broader than a single host. This spec adds a general and flexible scoping mechanism to server group policies which enables these use cases. Under this mechanism, "host" is just one of (potentially) many available scopes; by making host the default scope if none is specified, this mechanism is fully backward-compatible with the existing server group policies. Arbitrary additional scopes can be defined by the deployer.