fix: admin info output and improve overall performance (#10015)

- admin info node offline check is now quicker
- admin info now doesn't duplicate the code
  across doing the same checks for disks
- rely on StorageInfo to return appropriate errors
  instead of calling locally.
- diskID checks now return proper errors when
  disk not found v/s format.json missing.
- add more disk states for more clarity on the
  underlying disk errors.

pkg/madmin/heal-commands.go

@@ -1,5 +1,5 @@
 /*
- * MinIO Cloud Storage, (C) 2017, 2018 MinIO, Inc.
+ * MinIO Cloud Storage, (C) 2017-2020 MinIO, Inc.
  *
  * Licensed under the Apache License, Version 2.0 (the "License");
  * you may not use this file except in compliance with the License.
@@ -99,6 +99,9 @@ const (
 	DriveStateOffline            = "offline"
 	DriveStateCorrupt            = "corrupt"
 	DriveStateMissing            = "missing"
+	DriveStatePermission         = "permission-denied"
+	DriveStateFaulty             = "faulty"
+	DriveStateUnknown            = "unknown"
 	DriveStateUnformatted        = "unformatted" // only returned by disk
 )