Is it normal for air to be leaking from where the drain hose connects to the central A/C unit?
That picture is kind of unclear but it certainly looks to me as if the drain (and maybe the whole a/c) was added as a retrofit to a furnace - the hole is uneven looking to me - sloppy.  I have something similar myself, in fact.

In answer:  

1) No, it's not normal - it' a waste.  Air exchangers that are built to do both jobs do not have these kinds of gaps.

2) Yes - cut a piece of plastic to fit over the hole as much as possible.  Seal in place using METAL tape (not duct tape), closing up all the remaining gaps.