[#218] handler,s3-gw: Make policy configurable

Now default policy of placing containers can be set via config/env
variable.

Signed-off-by: Angira Kekteeva <kira@nspcc.ru>

api/handler/api.go

@@ -3,6 +3,7 @@ package handler
 import (
 	"errors"
 
+	"github.com/nspcc-dev/neofs-api-go/pkg/netmap"
 	"github.com/nspcc-dev/neofs-s3-gw/api"
 	"github.com/nspcc-dev/neofs-s3-gw/api/layer"
 	"go.uber.org/zap"
@@ -12,13 +13,22 @@ type (
 	handler struct {
 		log *zap.Logger
 		obj layer.Client
+		cfg *Config
+	}
+
+	// Config contains data which handler need to keep.
+	Config struct {
+		DefaultPolicy *netmap.PlacementPolicy
 	}
 )
 
+// DefaultPolicy is a default policy of placing container in NeoFS if it's not set at the request.
+const DefaultPolicy = "REP 3"
+
 var _ api.Handler = (*handler)(nil)
 
 // New creates new api.Handler using given logger and client.
-func New(log *zap.Logger, obj layer.Client) (api.Handler, error) {
+func New(log *zap.Logger, obj layer.Client, cfg *Config) (api.Handler, error) {
 	switch {
 	case obj == nil:
 		return nil, errors.New("empty NeoFS Object Layer")
@@ -29,5 +39,6 @@ func New(log *zap.Logger, obj layer.Client) (api.Handler, error) {
 	return &handler{
 		log: log,
 		obj: obj,
+		cfg: cfg,
 	}, nil
 }