Define 3 image clients as libraries

These 3 image clients have good interfaces now, then this patch makes
these clients as libraries.

Partially implements blueprint consistent-service-method-names

Change-Id: Iab90412ff4e793b0bb395b8ba8b6d35cef316dec
diff --git a/releasenotes/notes/image-clients-as-library-86d17caa26ce3961.yaml b/releasenotes/notes/image-clients-as-library-86d17caa26ce3961.yaml
new file mode 100644
index 0000000..46c0915
--- /dev/null
+++ b/releasenotes/notes/image-clients-as-library-86d17caa26ce3961.yaml
@@ -0,0 +1,9 @@
+---
+features:
+  - Define image service clients as libraries
+    The following image service clients are defined as library interface,
+    so the other projects can use these modules as stable libraries
+    without any maintenance changes.
+      **namespaces_client**
+      **resource_types_client**
+      **schemas_client**