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/fs-v1-metadata_test.go

@@ -73,7 +73,7 @@ func TestReadFSMetadata(t *testing.T) {
 	}
 
 	// Test with corrupted disk
-	fsStorage := fs.storage.(*posix)
+	fsStorage := fs.storage.(*retryStorage)
 	naughty := newNaughtyDisk(fsStorage, nil, errFaultyDisk)
 	fs.storage = naughty
 	if _, err := readFSMetadata(fs.storage, ".minio.sys", fsPath); errorCause(err) != errFaultyDisk {
@@ -111,7 +111,7 @@ func TestWriteFSMetadata(t *testing.T) {
 	}
 
 	// Reading metadata with a corrupted disk
-	fsStorage := fs.storage.(*posix)
+	fsStorage := fs.storage.(*retryStorage)
 	for i := 1; i <= 2; i++ {
 		naughty := newNaughtyDisk(fsStorage, map[int]error{i: errFaultyDisk, i + 1: errFaultyDisk}, nil)
 		fs.storage = naughty