Extend the locator function assert_rf_rack_valid_keyspace to accept
arbitrary topology dc-rack maps and nodes instead of using the current
token metadata.
This allows us to add a new variant of the function that checks rf-rack
validity given a topology change that we want to apply. we will use it
to check that rf-rack validity will be maintained before applying the
topology change.
The possible topology changes for the check are node add and node remove
/ decommission. These operations can change the number of normal racks -
if a new node is added to a new rack, or the last node is removed from a
rack.
9e1f78d162