Allow strict anti-affinity for servers at install time

Expose the option of strict anti-affinity (or any other Server group policy allowed in Nova) in install-config for the users to set at install-time.

Currently, the masters are deployed with a default policy of "soft-anti-affinity" and the workers are deployed without a server group.

The implementation should change the default for workers to be "soft-anti-affinity".

Both policies will be configured in the new MachinePool property serverGroupPolicy, in install-config.yaml.