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/endpoint.go

@@ -32,7 +32,6 @@ import (
 	"github.com/minio/cli"
 	"github.com/minio/minio-go/v6/pkg/set"
 	"github.com/minio/minio/cmd/config"
-	"github.com/minio/minio/cmd/config/etcd"
 	"github.com/minio/minio/cmd/logger"
 	"github.com/minio/minio/pkg/env"
 	"github.com/minio/minio/pkg/mountinfo"
@@ -590,10 +589,8 @@ func CreateEndpoints(serverAddr string, args ...[]string) (string, EndpointList,
 		return serverAddr, endpoints, setupType, err
 	}
 
-	_, dok := env.Lookup(config.EnvDomain)
-	_, eok := env.Lookup(etcd.EnvEtcdEndpoints)
-	_, iok := env.Lookup(config.EnvPublicIPs)
-	if dok && eok && !iok {
+	publicIPs := env.Get(config.EnvPublicIPs, "")
+	if len(publicIPs) == 0 {
 		updateDomainIPs(uniqueArgs)
 	}