container anti-affiniy policy

Like nova server group policies, container also has use case for anti-affinity policy.

As an Openstack administrator, I would like to distribute my two containers to different hosts for high availability reasons, such as my two mariadb container behind a haproxy , I would like to distribute the two mariadbs to different container hosts in case one host is down and the other one still can handle the request.

This blueprints propose to add the anti-affinity policies to zun. As the 'capsule' is some sort of the affinity implementation, so this blueprint just propose to add the anti-affinity policy. Maybe we can consolidate this later.