Use Agent OVO in agents_db and test_agents_db

Agent object has been merged [1].
This patch uses Agent object in agents_db and test_agents_db.

We also introduce a new function (get_agents_object) and keep
the old function (get_agents_db) for backward compatibility.

[1] https://review.openstack.org/#/c/297887/

Co-Authored-By: Nguyen Phuong An <AnNP@vn.fujitsu.com>
Change-Id: I4c4283cb1aa05d52dca00cc249e094ea7d55b1d3
Partially-Implements: blueprint adopt-oslo-versioned-objects-for-db

neutron/objects/db/api.py

@@ -37,9 +37,8 @@ def count(context, model, **kwargs):
 
 
 def _kwargs_to_filters(**kwargs):
-    return {k: v if (isinstance(v, list) or
-                     isinstance(v, obj_utils.StringMatchingFilterObj))
-            else [v]
+    retain_classes = (list, set, obj_utils.StringMatchingFilterObj)
+    return {k: v if isinstance(v, retain_classes) else [v]
             for k, v in kwargs.items()}