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/metrics.go

@@ -83,9 +83,9 @@ func (c *minioCollector) Collect(ch chan<- prometheus.Metric) {
 	minioVersionInfo.WithLabelValues(Version, CommitID).Set(float64(1.0))
 
 	// Fetch disk space info
-	objLayer := newObjectLayerFn()
+	objLayer := globalObjectAPI
 	// Service not initialized yet
-	if objLayer == nil {
+	if objLayer == nil || globalSafeMode {
 		return
 	}