With SERVER-87085, custom defaultWriteConcern values not known to the config server will be rejected, even if they might be available on the shards.
Since a custom write concern can only be specified at the level of individual shards, all custom values for the write concern used on the shards must be provided to the config server though its configuration.
Moreover, those values must match the values on the shard which might not be necessarily correct: tags might vary between CSRS and shards.

The goal of this ticket is to introduce a mechanism allowing to specify the tags & custom write concern at the cluster level.