nsqd: extend --tls-required to allow non-tls on the http port

This allows users to require tls on the https/tcp ports, but to
bypass tls on the http port.

Somebody could set up nsqd on a server, expose the tcp/https ports,
lockdown the http port, and allow localhost programs to post to nsqd
without adding support for certs/ssl.
Yeah, the idiom exists (in effective go and maps in action), exists is just more semantically correct to my mind. I can change it if you want.