"be to be happy" is referencing the future, a state that doesn't exist yet. It's similar to "will be happy" and "will become happy". People who are to be happy... People who will be happy... People who will become happy... But there is also the feeling of "in order to be" in that expression. People who are to be happy... = In order for people to end up being happy, they should... = If people want to be happy, they should...

"People who are to be happy fix their attention on the convenience of thing." is a confusing sentence. "Are to be happy" is not good English usage. I can't think of any reason to use it. "The convenience of thing" is meaningless. A correct sentence might read, "People who are trying (or determined, etc.)to be happy fix their attention on ... (some goal or method)."

Mojave's answer was perfect