Currently the shell helpers are called addUser and addRole, but the commands they use are called createUser and createRole. We should make these names consistent.

While addUser is what we had in the past, I think createUser is the better name as it's more standard in databases, and more consistent with other creation operations in the system, like createCollection and createIndex.

Also, by moving away from addUser we can change the arguments so that write concern is passed to createUser as an object rather than having "numReplicatedTo" and "timeout" fields.