Fix query in get_l3_agent_with_min_routers

For PostgreSQL if you're using GROUP BY everything in the SELECT
list must be an aggregate count(...) or used in the GROUP BY.

Closes-bug: #1495523

Change-Id: Ieb75d0666ec2f6d2e61686bf2bacea2b9ad6c521

neutron/db/l3_agentschedulers_db.py

@@ -501,6 +501,7 @@ class L3AgentSchedulerDbMixin(l3agentscheduler.L3AgentSchedulerPluginBase,
             func.count(
                 RouterL3AgentBinding.router_id
             ).label('count')).outerjoin(RouterL3AgentBinding).group_by(
+                agents_db.Agent.id,
                 RouterL3AgentBinding.l3_agent_id).order_by('count')
         res = query.filter(agents_db.Agent.id.in_(agent_ids)).first()
         return res[0]