Monitor should allow empty username and password

[xiaosuiba]

What happened:
monitor does not allow empty username and password for elasticsearch storage type.

What you expected to happen:
could use elasticsearch storage type monitor without username and password, like any other settings related to elasticsearch.

How to reproduce it (as minimally and precisely as possible):

1. install a new tke with elasticsearch as monitor storage backend.
2. do not provide username and password

Anything else we need to know?:

type ESMonitor struct {
	URL      string `json:"url" validate:"required"`
	Username string `json:"username" validate:"required"`
	Password []byte `json:"password" validate:"required"`
}

Environment:

• TKE version: v1.6.0
• Global or business cluster: global
• Kubernetes version (use kubectl version):
• Install addons:
• Others: