Mark credentials_factory stable for plugins

Add unit tests for credentials_factory and mark it stable for
plugins. This is part of the work to make tempest/test.py stable
for plugins.

Change-Id: I99414cb2704ce5469fcb66406b6905e2d1871dba
diff --git a/releasenotes/notes/credentials-factory-stable-c8037bd9ae642482.yaml b/releasenotes/notes/credentials-factory-stable-c8037bd9ae642482.yaml
new file mode 100644
index 0000000..6faa536
--- /dev/null
+++ b/releasenotes/notes/credentials-factory-stable-c8037bd9ae642482.yaml
@@ -0,0 +1,10 @@
+---
+features:
+  - |
+    The credentials_factory.py module is now marked as stable for Tempest
+    plugins. It provides helpers that can be used by Tempest plugins to
+    obtain test credentials for their test cases in a format that honors the
+    Tempest configuration in use.
+    Credentials may be provisioned on the fly during the test run, or they
+    can be setup in advance and fed to test via a YAML file; they can be
+    setup for identity v2 or identity v3.