Use all available headroom for autoscaling

Fixes bug 1254796

Previously, potential headroom for autoscaling group growth
or shrinkage would remain unused unless the adjustment exactly
hit the max or min size respectively.

Now, we truncate the adjustment to the upper or lower bound
in the case of an over-shoot.

Change-Id: I760be999039f7038dab77cb8b77a0f683ef56db3