Python 3: use next() instead of iterator.next()

The latter only works in Python 2.

Also define a __next__ method in the classes that define a next method.

Change-Id: Iaa1a1e500facab50d8bcdffda39ccad3f2e4e9bb
Blueprint: neutron-python3
diff --git a/neutron/tests/tempest/common/glance_http.py b/neutron/tests/tempest/common/glance_http.py
index 6cdbadc..0a6f985 100644
--- a/neutron/tests/tempest/common/glance_http.py
+++ b/neutron/tests/tempest/common/glance_http.py
@@ -367,7 +367,7 @@
 
     def __iter__(self):
         while True:
-            yield self.next()
+            yield next(self)
 
     def next(self):
         chunk = self.resp.read(CHUNKSIZE)
@@ -375,3 +375,5 @@
             return chunk
         else:
             raise StopIteration()
+
+    __next__ = next