LBaaS: Set correct nullable parameter for agent_id
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

gandelman-a/neutron-lbaas,gotostack/neutron-lbaas,gandelman-a/neutron-lbaas,gotostack/neutron-lbaas,citrix-openstack-build/neutron-lbaas,gandelman-a/neutron-lbaas,dougwig/x-neutron-lbaas,gotostack/neutron-lbaas