Separate keyspace which also behaves as system brings
little benefit while creating some compatibility problems
like schema digest mismatch during rollback. So we decided
to move auth tables into system keyspace.
Fixes https://github.com/scylladb/scylladb/issues/18098
Closes scylladb/scylladb#18769
(cherry picked from commit 2ab143fb40)
[avi: adjust test/alternator/suite.yaml to reflect new keyspace]