Mising CLI test for Keystone

This patch adds following missing Keystone CLI test -
1. ec2 credentialslist

Partially implements: blueprint missing-cli-tests-in-tempest

Change-Id: Ibf0070e12f3087756addcbed841d52c73cf70553
diff --git a/tempest/cli/simple_read_only/test_keystone.py b/tempest/cli/simple_read_only/test_keystone.py
index 1efbede..dda65c1 100644
--- a/tempest/cli/simple_read_only/test_keystone.py
+++ b/tempest/cli/simple_read_only/test_keystone.py
@@ -117,6 +117,11 @@
     def test_admin_bashcompletion(self):
         self.keystone('bash-completion')
 
+    def test_admin_ec2_credentials_list(self):
+        creds = self.keystone('ec2-credentials-list')
+        creds = self.parser.listing(creds)
+        self.assertTableStruct(creds, ['tenant', 'access', 'secret'])
+
     # Optional arguments:
 
     def test_admin_version(self):