Add namespace properties client and tests

As in the doc following, there are namespace properties apis, this patch
is to add them to the code.

[doc]http://developer.openstack.org/api-ref/image/v2/metadefs-index.html

Change-Id: I593c72cea6ed1381d0ddef1569d40e65ae45625b
diff --git a/releasenotes/notes/add-image-clients-af94564fb34ddca6.yaml b/releasenotes/notes/add-image-clients-af94564fb34ddca6.yaml
new file mode 100644
index 0000000..7e40fd4
--- /dev/null
+++ b/releasenotes/notes/add-image-clients-af94564fb34ddca6.yaml
@@ -0,0 +1,9 @@
+---
+features:
+  - |
+    As in the [doc]:
+    http://developer.openstack.org/api-ref/image/v2/metadefs-index.html,
+    there are some apis are not included, add them.
+
+      * namespace_properties_client(v2)
+