Replace 'raise StopIteration' with 'return'
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

openstack/python-glanceclient,openstack/python-glanceclient