Is it common for a chimney to separate from the outside wall?
This is common but its also a big issue.  I doubt that this has to do with your house foundation although it could be a factor.  The main cause would be a weak footing below your chimney.  

The fact that you are having a dry year might have caused the footings to move earlier but if they weren't up to spec then it was going to move.  You have to figure that your chimney may weigh a few tons.  It has the house on one side.  So the chimney is going to move away from pressure if the footings are weak.  So that is why the top is separating probably a little more than the bottom.

I am not sure exactly who was supposed to do what to make the chimney 7 years ago.... But normally the mason would create proper footings or at least not build until it was done right.  If your mason was contracted to do everything he messed up or your soil is completely unstable (which he should have researched before doing the job).  

So now you need to get this fixed ASAP.  If it hasn't had too much damage they can jack it back into place, repair footings, and bracket it into place.  This is much much cheaper than tear down and build.  If you have a warranty you will need to keep trying to contact the mason - maybe he gives you a deal on the repair.  Fixing this yourself really isn't DIY because the tools/equipment are too costly for a one time job.