[PROPOSAL] Rename ssl_server_name.yaml to sni.yaml

[Randall Meyer]

I'd like to rename ssl_server_name.yaml to sni.yaml for ATS 9. This would also 
affect the toplevel key in the yaml config file (making the contents rooted 
under "sni"). The code/classes it is used to configure is SNIConfig. And 
personally, I find the current name to be a bit of a mouthful.