[#172] Convert handler config to interface

Signed-off-by: Marina Biryukova <m.biryukova@yadro.com>

api/handler/api_test.go

@@ -12,11 +12,9 @@ func TestCopiesNumberPicker(t *testing.T) {
 	locationConstraint2 := "two"
 	locationConstraints[locationConstraint1] = []uint32{2, 3, 4}
 
-	config := &Config{
-		Policy: &placementPolicyMock{
-			copiesNumbers:        locationConstraints,
-			defaultCopiesNumbers: []uint32{1},
-		},
+	config := &configMock{
+		copiesNumbers:        locationConstraints,
+		defaultCopiesNumbers: []uint32{1},
 	}
 	h := handler{
 		cfg: config,