objectLayer: Check for format.json in a wrapped disk. (#3311)

This is needed to validate if the `format.json` indeed exists
when a fresh node is brought online.

This wrapped implementation also connects to the remote node
by attempting a re-login. Subsequently after a successful
connect `format.json` is validated as well.

Fixes #3207

cmd/storage-interface.go

@@ -24,6 +24,8 @@ type StorageAPI interface {
 	String() string
 
 	// Storage operations.
+	Init() (err error)
+	Close() (err error)
 	DiskInfo() (info disk.Info, err error)
 
 	// Volume operations.