Add a command-line switch to enable DoH without HTTPS

We want to set a load balance and put the dnsproxy behead it, only put the HTTPS certificate on load balance.
Found that we can't start dnsproxy with no TLS config:
![image](https://user-images.githubusercontent.com/16044327/119986020-f26c2b00-bff5-11eb-8f29-6c9ec29f6e86.png)
any plan to support this scenario?
