Move cred_client to tempest.lib

This commit migrates the cred_client module from tempest.common to
tempest lib. This module provides an abstraction around the differences
between v2 credential operations and v3 credential operations. Since for
a lot of tests you just need to interact with credentials and which
version of keystone is deployed or being used doesn't matter. As part of
this migration unit tests are added to handle testing the abstraction
layer.

Change-Id: I196033483e06c96c16eaefeaac68b3253c3347f5
diff --git a/releasenotes/notes/add-cred_client-to-tempest.lib-4d4af33f969c576f.yaml b/releasenotes/notes/add-cred_client-to-tempest.lib-4d4af33f969c576f.yaml
new file mode 100644
index 0000000..432a6b1
--- /dev/null
+++ b/releasenotes/notes/add-cred_client-to-tempest.lib-4d4af33f969c576f.yaml
@@ -0,0 +1,5 @@
+---
+features:
+  - The cred_client module was added to tempest.lib. This module provides a
+    wrapper to the keystone services client which provides a uniform
+    interface that abstracts out the differences between keystone api versions.