Fix Requests breaking download progress bar

The move to the requests library (dbb242b) broke the progress bar during
downloading an image with --progress enabled, due to requests returning
a generator, which has no __len__ function. This patch adds an iterable
wrapper which provides the length, sourced from Content-Length headers.

Closes-Bug: #1384664

Change-Id: I48598a824396bc6e7cba69eb3ce32e7c8f30a18a

glanceclient/common/utils.py

@@ -421,3 +421,18 @@ def safe_header(name, value):
         return name, "{SHA1}%s" % d
     else:
         return name, value
+
+
+class IterableWithLength(object):
+    def __init__(self, iterable, length):
+        self.iterable = iterable
+        self.length = length
+
+    def __iter__(self):
+        return self.iterable
+
+    def next(self):
+        return next(self.iterable)
+
+    def __len__(self):
+        return self.length