server: Implement --ignore-disks for ignoring disks from healing. (#2158)

By default server heals/creates missing directories and re-populates
`format.json`, in some scenarios when disk is down for maintainenance
it would be beneficial for users to ignore such disks rather than
mistakenly using `root` partition.

Fixes #2128

xl-v1_test.go

@@ -132,7 +132,7 @@ func TestNewXL(t *testing.T) {
 		defer removeAll(disk)
 	}
 	// Initializes all erasure disks
-	_, err := newXLObjects(erasureDisks)
+	_, err := newXLObjects(erasureDisks, nil)
 	if err != nil {
 		t.Fatalf("Unable to initialize erasure, %s", err)
 	}