SERVER-9516 Make getUserDescription in mongod look in the right collection during upgrade.
SERVER-9516 Stop doing system.users index upgrade implicitly.

The right place to upgrade the admin.system.users indexes is during the explicit
2.4->2.6 upgrade process, not on opening the admin database as it was in the
2.2->2.4 upgrade process.

Furthermore, since system indexes are only created on system collection creation, now,
there's no need to check the auth schema version when creating indexes on admin.system.users.
If the collection did not exist before, the schema version must be 2.6.
SERVER-9516 Update admin.system.version as part of user management commands.