Make argument params of list methods consistent

The argument type "params" is not consistent between list methods of
compute service clients. This patch makes them consistent.

Partially implements blueprint consistent-service-method-names

Change-Id: I40ca879f2c8b82871d4a9bc1592a5396cf00aeb8
diff --git a/tempest/services/compute/json/agents_client.py b/tempest/services/compute/json/agents_client.py
index e0e3ec3..ebc6809 100644
--- a/tempest/services/compute/json/agents_client.py
+++ b/tempest/services/compute/json/agents_client.py
@@ -25,7 +25,7 @@
     Tests Agents API
     """
 
-    def list_agents(self, params=None):
+    def list_agents(self, **params):
         """List all agent builds."""
         url = 'os-agents'
         if params: