refactor TLS warning messages

[mario-mazo]

TLS warning messages should be refactored to a more flexible approach

Expected Behaviour

something like

checkTLS(io.Writer, gateway, tlsenabled)

should validate and print the warning

Current Behaviour

There is a return statement and extra \n

Possible Solution

please refer to #581

[mario-mazo]

I will work on it

[alexellis]

Putting this into review.