Replace 'raise StopIteration' with 'return'

With PEP 479, the behaviour of StopIteration is changing. Raising it to
stop a generator is considered incorrect and from Python 3.7 this will
cause a RuntimeError. The PEP recommends using the return statement.

More details: https://www.python.org/dev/peps/pep-0479/#examples-of-breakage

Change-Id: Ia067940066a5666926dcf61136b03d721a87666e

glanceclient/v2/images.py

@@ -125,7 +125,7 @@ class Controller(object):
                     if limit:
                         limit -= 1
                         if limit <= 0:
-                            raise StopIteration
+                            return
 
                 try:
                     next_url = body['next']