locator: Move rack_list to topology.hh

So that we can use it in locator/tablets.hh and avoid circular dependency
between that header and abstract_replication_strategy.hh

locator/abstract_replication_strategy.hh

@@ -45,8 +45,6 @@ enum class replication_strategy_type {
     everywhere_topology,
 };
 
-using rack_list = std::vector<sstring>;
-
 using replication_strategy_config_option = std::variant<sstring, rack_list>;
 using replication_strategy_config_options = std::map<sstring, replication_strategy_config_option>;
 

locator/topology.hh

@@ -40,6 +40,8 @@ using node_holder = std::unique_ptr<node>;
 
 using shard_id = seastar::shard_id;
 
+using rack_list = std::vector<sstring>;
+
 class node {
 public:
     using this_node = bool_class<struct this_node_tag>;