A hiring manager will not want to hire you if they think that you will not stay in the position for very long. Assure the interviewer that hiring and training you would be worth the investment of time.
"I see myself working as a FA for the very long term. Travel is a passion of mine and working for an airline would make this passion become a reality."
"I want to be a flight attendant for as long as I can. There's nothing more exciting to me than traveling and taking care of people. I've been building my lifestyle around this kind of work ever since I made that decision to be a flight attendant. I'm committed to it, and happily so."