config: Migrate to the new version. Remove backend details.

Migrate to new config format v4.
```
{
	"version": "4",
	"credential": {
		"accessKey": "WLGDGYAQYIGI833EV05A",
		"secretKey": "BYvgJM101sHngl2uzjXS/OBF/aMxAN06JrJ3qJlF"
	},
	"region": "us-east-1",
	"logger": {
		"console": {
			"enable": true,
			"level": "fatal"
		},
		"file": {
			"enable": false,
			"fileName": "",
			"level": "error"
		},
		"syslog": {
			"enable": false,
			"address": "",
			"level": "debug"
		}
	}
}
```

This patch also updates [minio cli spec](./minio.md)

config-v4.go

@@ -0,0 +1,202 @@
+/*
+ * Minio Cloud Storage, (C) 2016 Minio, Inc.
+ *
+ * Licensed under the Apache License, Version 2.0 (the "License");
+ * you may not use this file except in compliance with the License.
+ * You may obtain a copy of the License at
+ *
+ *     http://www.apache.org/licenses/LICENSE-2.0
+ *
+ * Unless required by applicable law or agreed to in writing, software
+ * distributed under the License is distributed on an "AS IS" BASIS,
+ * WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+ * See the License for the specific language governing permissions and
+ * limitations under the License.
+ */
+
+package main
+
+import (
+	"os"
+	"sync"
+
+	"github.com/minio/minio/pkg/probe"
+	"github.com/minio/minio/pkg/quick"
+)
+
+// serverConfigV4 server configuration version '4'.
+type serverConfigV4 struct {
+	Version string `json:"version"`
+
+	// S3 API configuration.
+	Credential credential `json:"credential"`
+	Region     string     `json:"region"`
+
+	// Additional error logging configuration.
+	Logger logger `json:"logger"`
+
+	// Read Write mutex.
+	rwMutex *sync.RWMutex
+}
+
+// initConfig - initialize server config. config version (called only once).
+func initConfig() *probe.Error {
+	if !isConfigFileExists() {
+		srvCfg := &serverConfigV4{}
+		srvCfg.Version = globalMinioConfigVersion
+		srvCfg.Region = "us-east-1"
+		srvCfg.Credential = mustGenAccessKeys()
+		srvCfg.rwMutex = &sync.RWMutex{}
+		// Create config path.
+		err := createConfigPath()
+		if err != nil {
+			return err.Trace()
+		}
+
+		// Create certs path.
+		err = createCertsPath()
+		if err != nil {
+			return err.Trace()
+		}
+
+		// Save the new config globally.
+		serverConfig = srvCfg
+
+		// Save config into file.
+		err = serverConfig.Save()
+		if err != nil {
+			return err.Trace()
+		}
+		return nil
+	}
+	configFile, err := getConfigFile()
+	if err != nil {
+		return err.Trace()
+	}
+	if _, e := os.Stat(configFile); err != nil {
+		return probe.NewError(e)
+	}
+	srvCfg := &serverConfigV4{}
+	srvCfg.Version = globalMinioConfigVersion
+	srvCfg.rwMutex = &sync.RWMutex{}
+	qc, err := quick.New(srvCfg)
+	if err != nil {
+		return err.Trace()
+	}
+	if err := qc.Load(configFile); err != nil {
+		return err.Trace()
+	}
+	// Save the loaded config globally.
+	serverConfig = qc.Data().(*serverConfigV4)
+	// Set the version properly after the unmarshalled json is loaded.
+	serverConfig.Version = globalMinioConfigVersion
+	return nil
+}
+
+// serverConfig server config.
+var serverConfig *serverConfigV4
+
+// GetVersion get current config version.
+func (s serverConfigV4) GetVersion() string {
+	s.rwMutex.RLock()
+	defer s.rwMutex.RUnlock()
+	return s.Version
+}
+
+/// Logger related.
+
+// SetFileLogger set new file logger.
+func (s *serverConfigV4) SetFileLogger(flogger fileLogger) {
+	s.rwMutex.Lock()
+	defer s.rwMutex.Unlock()
+	s.Logger.File = flogger
+}
+
+// GetFileLogger get current file logger.
+func (s serverConfigV4) GetFileLogger() fileLogger {
+	s.rwMutex.RLock()
+	defer s.rwMutex.RUnlock()
+	return s.Logger.File
+}
+
+// SetConsoleLogger set new console logger.
+func (s *serverConfigV4) SetConsoleLogger(clogger consoleLogger) {
+	s.rwMutex.Lock()
+	defer s.rwMutex.Unlock()
+	s.Logger.Console = clogger
+}
+
+// GetConsoleLogger get current console logger.
+func (s serverConfigV4) GetConsoleLogger() consoleLogger {
+	s.rwMutex.RLock()
+	defer s.rwMutex.RUnlock()
+	return s.Logger.Console
+}
+
+// SetSyslogLogger set new syslog logger.
+func (s *serverConfigV4) SetSyslogLogger(slogger syslogLogger) {
+	s.rwMutex.Lock()
+	defer s.rwMutex.Unlock()
+	s.Logger.Syslog = slogger
+}
+
+// GetSyslogLogger get current syslog logger.
+func (s *serverConfigV4) GetSyslogLogger() syslogLogger {
+	s.rwMutex.RLock()
+	defer s.rwMutex.RUnlock()
+	return s.Logger.Syslog
+}
+
+// SetRegion set new region.
+func (s *serverConfigV4) SetRegion(region string) {
+	s.rwMutex.Lock()
+	defer s.rwMutex.Unlock()
+	s.Region = region
+}
+
+// GetRegion get current region.
+func (s serverConfigV4) GetRegion() string {
+	s.rwMutex.RLock()
+	defer s.rwMutex.RUnlock()
+	return s.Region
+}
+
+// SetCredentials set new credentials.
+func (s *serverConfigV4) SetCredential(creds credential) {
+	s.rwMutex.Lock()
+	defer s.rwMutex.Unlock()
+	s.Credential = creds
+}
+
+// GetCredentials get current credentials.
+func (s serverConfigV4) GetCredential() credential {
+	s.rwMutex.RLock()
+	defer s.rwMutex.RUnlock()
+	return s.Credential
+}
+
+// Save config.
+func (s serverConfigV4) Save() *probe.Error {
+	s.rwMutex.RLock()
+	defer s.rwMutex.RUnlock()
+
+	// get config file.
+	configFile, err := getConfigFile()
+	if err != nil {
+		return err.Trace()
+	}
+
+	// initialize quick.
+	qc, err := quick.New(&s)
+	if err != nil {
+		return err.Trace()
+	}
+
+	// Save config file.
+	if err := qc.Save(configFile); err != nil {
+		return err.Trace()
+	}
+
+	// Return success.
+	return nil
+}