LBaaS: Set correct nullable parameter for agent_id

Corrected the following model according to database
content:
 - NOT NULL for PoolLoadbalancerAgentBinding agent_id;

Partial-bug: #1296282

Change-Id: I1f11ec2e8f4c16f58dd8d4e6429e46eaedaec945

neutron/services/loadbalancer/agent_scheduler.py

@@ -39,7 +39,8 @@ class PoolLoadbalancerAgentBinding(model_base.BASEV2):
                         primary_key=True)
     agent = orm.relation(agents_db.Agent)
     agent_id = sa.Column(sa.String(36), sa.ForeignKey("agents.id",
-                                                      ondelete='CASCADE'))
+                                                      ondelete='CASCADE'),
+                         nullable=False)
 
 
 class LbaasAgentSchedulerDbMixin(agentschedulers_db.AgentSchedulerDbMixin,