Add test list namespace function

Both test_images.py and test_images_member.py have test list funtion,
we can also add a function in test_images_metadefs_namespaces.py for
testing namespaces list.

Another reason is we have done create, get detail, update and delete
namespace test. But did not test the list.

Related link:
http://developer.openstack.org/api-ref/image/v2/metadefs-index.html

Change-Id: I4caaccec9dea29f6a368f013a3edaad893fafada
2 files changed