XL/Erasure: Make bit-rot verification based on xl.json algo. (#2299)

Currently `xl.json` saves algorithm information for bit-rot
verification. Since the bit-rot algo's can change in the
future make sure the erasureReadFile doesn't default to
a particular algo. Instead use the checkSumInfo.

object-api-putobject_test.go

@@ -199,7 +199,7 @@ func testObjectAPIPutObjectDiskNotFOund(obj ObjectLayer, instanceType string, di
 	}
 
 	// Take 8 disks down, one more we loose quorum on 16 disk node.
-	for _, disk := range disks[:8] {
+	for _, disk := range disks[:7] {
 		removeAll(disk)
 	}