Cannot use password without tls enabled in email type notify channel

[xiaosuiba]

What happened: If I create an email type notify channel without tls enabled, the notify-controller will reject the configuration and complain

sendMail error: unencrypted connection

What you expected to happen: enable sending email without tls enabled or force to use tls.

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

Anything else we need to know?: http://golang.org/src/pkg/net/smtp/auth.go?s=1820:1882#L41

Environment:

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