turn on warning-is-error in sphinx build

Fix a formatting error in one docstring and turn on the flag to ensure
that future warnings in the doc build trigger a build failure.

Change-Id: I7159b985d1690a8ae61ff885408da4623c105952
Signed-off-by: Doug Hellmann <doug@doughellmann.com>

glanceclient/common/utils.py

@@ -311,7 +311,7 @@ def get_file_size(file_obj):
     """Analyze file-like object and attempt to determine its size.
 
     :param file_obj: file-like object.
-    :retval The file's size or None if it cannot be determined.
+    :retval: The file's size or None if it cannot be determined.
     """
     if (hasattr(file_obj, 'seek') and hasattr(file_obj, 'tell') and
             (six.PY2 or six.PY3 and file_obj.seekable())):