Skip bindings with agent_id=None

Skip bindings with agent_id=None

To avoid having extra L3HARouterAgentPortBinding with l3_agent as None,
operation of setting l3_agent should be atomic.
For this purpose, transaction was added in methods
create_ha_port_and_bind and _bind_ha_router_to_agents in change
Iaad82fe522cfd70061daecf411c924fdc11b7e41

In case if router was just created and l3 agent was not scheduled yet,
so l3_agent_id is None, l3-agent-list-hosting-router <router> will fail.
This change makes it work by skipping binding with agent_id=None.

Partial-bug: #1499647

Change-Id: I1aaf4b651f738febc26b0e1105aeabe066bca2a0