mongos cannot implicitly create the shard key index when featureCompatibilityVersion is 3.2

After SERVER-25156 is implemented, specifying the "collation" option in an index specification will require building a v=2 index. However, building a v=2 index is disallowed when the server's featureCompatibilityVersion is 3.2. We should allow mongos to still be able to implicitly create the shard key index on collections with a "simple" default collation by not passing {locale: "simple"} to clusterCreateIndex().