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/web-router.go

@@ -62,8 +62,18 @@ const specialAssets = "index_bundle.*.js|loader.css|logo.svg|firefox.png|safari.
 func registerWebRouter(router *mux.Router) error {
 	// Initialize Web.
 	web := &webAPIHandlers{
-		ObjectAPI: newObjectLayerFn,
-		CacheAPI:  newCacheObjectsFn,
+		ObjectAPI: func() ObjectLayer {
+			if !globalSafeMode {
+				return globalObjectAPI
+			}
+			return nil
+		},
+		CacheAPI: func() CacheObjectLayer {
+			if !globalSafeMode {
+				return globalCacheObjectAPI
+			}
+			return nil
+		},
 	}
 
 	// Initialize a new json2 codec.