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
Merge "LBaaS: Set correct nullable parameter for agent_id"