Server Group Anti Affinity for OpenStack DBaaS (Trove)

add server group anti affinity for clusters + read-replicas + possibly adhoc.

Clusters Config and Router Flavor Sizing for OpenStack DBaaS (Trove)

As it stands now, the config-server and query router flavor matches that of the member. As pointed out at https://groups.google.com/forum/#!msg/mongodb-user/Epi10dVzoyo/hPJGy-HCKrUJ and various other places, the config-server does not require such a large flavor. It looks like we can introduce a CONF.mongodb.config_...