Join tables in query for down L3 agents

Join the agent table together with the L3 agent scheduler
table. This fixes an issue where the query would return
all admin_state_up L3 agents with router bindings when
any agent in the database was dead (missed heartbeats)
instead of just returning the dead L3 agents with bindings.

Closes-Bug: #1359460
Change-Id: I29215f81be7e3513155faaad015e24dbf1a1ad5d
Merge "Join tables in query for down L3 agents"