Copy the keyframe in your first frame, and paste it on the very last frame where you want your cycle to come back to its starting point. (I originally made mine a mere 30 frames long total, but I found that it was far too fast and ended up extending it to over 150 frames.) Then paste it again--this time at the midpoint of your timeline, halfway between the beginning and ending frames.
On that bisecting middle frame, go ahead and select your clouds to move them around, to the farthest point that you want them to glide out to.
Then right-click on your timeline between your first and second keyframes, and select Create Motion Tween. Create a second motion tween between the second and third keyframes.
What this does is create a cyclical motion; the clouds glide gently to one side, then gently back, by tweening from the starting position to the ending position and then back again. Because it's contained inside a movie clip with no actions controlling its default behavior, the animation will play constantly in the background with no further controls required.