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/policy.go

@@ -69,7 +69,7 @@ func (sys *PolicySys) IsAllowed(args policy.Args) bool {
 	if globalIsGateway {
 		// When gateway is enabled, no cached value
 		// is used to validate bucket policies.
-		objAPI := newObjectLayerFn()
+		objAPI := globalObjectAPI
 		if objAPI != nil {
 			config, err := objAPI.GetBucketPolicy(context.Background(), args.BucketName)
 			if err == nil {