User name argument in auth_method breaks Cartridge

Commit 459f7ec5 introduced a new user name argument in the function box.schema.user.password(..). This is a public API used by cartridge internals. And as a result the Cartridge is broken with errors like:

...rocks/share/tarantool/cartridge/test-helpers/cluster.lua:209: "localhost:13303": ...est_app/.rocks/share/tarantool/cartridge/confapplier.lua:535: bad argument #3 to 'password' (string expected, got nil)