Add keystone v3 EC2 credential tests and client

The v2 version of the /users/{}/credentials/OS-EC2 API was already
present, for completeness this patch adds the v3 equivalent.

This API is not documented because it is preferred to use the
/v3/credentials API for this functionality.

Change-Id: Ide90658fa07a3d5a7ba2e52c3ab7377170273b76
diff --git a/releasenotes/notes/add-keystone-v3-ec2-tests-d959b7d36f0bd7fc.yaml b/releasenotes/notes/add-keystone-v3-ec2-tests-d959b7d36f0bd7fc.yaml
new file mode 100644
index 0000000..ab8d748
--- /dev/null
+++ b/releasenotes/notes/add-keystone-v3-ec2-tests-d959b7d36f0bd7fc.yaml
@@ -0,0 +1,5 @@
+---
+features:
+  - |
+    Added missing clients and tests for keystone's v3 EC2 API which already
+    existed for keystone v2.