Add functionality to add old buckets to etcd on startup

Buckets already present on a Minio server before it joins a
bucket federated deployment will now be added to etcd during
startup. In case of a bucket name collision, admin is informed
via Minio server console message.

Added configuration migration for configuration stored in etcd
backend.

Also, environment variables are updated and ListBucket path style
request is no longer forwarded.

cmd/config-current.go

@@ -127,10 +127,28 @@ func (s *serverConfig) GetCacheConfig() CacheConfig {
 	return s.Cache
 }
 
-// Save config.
-func (s *serverConfig) Save(configFile string) error {
-	// Save config file.
-	return quick.Save(configFile, s)
+// Save config file to corresponding backend
+func Save(configFile string, data interface{}) error {
+	if globalEtcdClient == nil {
+		return quick.SaveLocalConfig(configFile, data)
+	}
+	return quick.SaveEtcdConfig(configFile, data, globalEtcdClient)
+}
+
+// Load config from backend
+func Load(configFile string, data interface{}) (quick.Config, error) {
+	if globalEtcdClient == nil {
+		return quick.LoadLocalConfig(configFile, data)
+	}
+	return quick.LoadEtcdConfig(configFile, data, globalEtcdClient)
+}
+
+// GetVersion gets config version from backend
+func GetVersion(configFile string) (string, error) {
+	if globalEtcdClient == nil {
+		return quick.GetLocalVersion(configFile)
+	}
+	return quick.GetEtcdVersion(configFile, globalEtcdClient)
 }
 
 // Returns the string describing a difference with the given
@@ -272,7 +290,7 @@ func newConfig() error {
 	globalServerConfigMu.Unlock()
 
 	// Save config into file.
-	return globalServerConfig.Save(getConfigFile())
+	return Save(getConfigFile(), globalServerConfig)
 }
 
 // newQuickConfig - initialize a new server config, with an allocated