Extend further validation of config values (#8469)

- This PR allows config KVS to be validated properly
  without being affected by ENV overrides, rejects
  invalid values during set operation

- Expands unit tests and refactors the error handling
  for notification targets, returns error instead of
  ignoring targets for invalid KVS

- Does all the prep-work for implementing safe-mode
  style operation for MinIO server, introduces a new
  global variable to toggle safe mode based operations
  NOTE: this PR itself doesn't provide safe mode operations

cmd/globals.go

@@ -251,6 +251,11 @@ var (
 	globalBackgroundHealRoutine *healRoutine
 	globalBackgroundHealState   *allHealState
 
+	// Only enabled when one of the sub-systems fail
+	// to initialize, this allows for administrators to
+	// fix the system.
+	globalSafeMode bool
+
 	// Add new variable global values here.
 )