database: modify mongoc_database_add_user() signature for forward compat.

Let's change this before we move forward. We have not yet implemented
CDRIVER-232, but we can stub out for it anyway.

Legacy add_user test fails on 2.5.4, so a suplimental commit will
check for that.
cluster: perform auth after isMaster for replicaSet members.
cluster: perform auth after isMaster for sharded clusters.
tests: check for 2.5.x and ignore addUser.

Once we have an implementation for this, we can re-enable checks.