Refactor config and split them in packages (#8351)

This change is related to larger config migration PR
change, this is a first stage change to move our
configs to `cmd/config/` - divided into its subsystems

cmd/gateway-common.go

@@ -1,5 +1,5 @@
 /*
- * MinIO Cloud Storage, (C) 2017 MinIO, Inc.
+ * MinIO Cloud Storage, (C) 2017-2019 MinIO, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -18,11 +18,12 @@ package cmd
 
 import (
 	"net/http"
-	"os"
 	"strings"
 
+	"github.com/minio/minio/cmd/config"
 	xhttp "github.com/minio/minio/cmd/http"
 	"github.com/minio/minio/cmd/logger"
+	"github.com/minio/minio/pkg/env"
 	"github.com/minio/minio/pkg/hash"
 	xnet "github.com/minio/minio/pkg/net"
 
@@ -365,14 +366,14 @@ func parseGatewaySSE(s string) (gatewaySSE, error) {
 			gwSlice = append(gwSlice, v)
 			continue
 		}
-		return nil, uiErrInvalidGWSSEValue(nil).Msg("gateway SSE cannot be (%s) ", v)
+		return nil, config.ErrInvalidGWSSEValue(nil).Msg("gateway SSE cannot be (%s) ", v)
 	}
 	return gatewaySSE(gwSlice), nil
 }
 
 // handle gateway env vars
 func handleGatewayEnvVars() {
-	gwsseVal, ok := os.LookupEnv("MINIO_GATEWAY_SSE")
+	gwsseVal, ok := env.Lookup("MINIO_GATEWAY_SSE")
 	if ok {
 		var err error
 		GlobalGatewaySSE, err = parseGatewaySSE(gwsseVal)