Join tables in query for down L3 agents

Join the agent table together with the L3 agent scheduler
table based on the agent ID so the down L3 agent query
doesn't incorrectly return L3 agents when any agent is down.

Closes-Bug: #1359460
Change-Id: I29215f81be7e3513155faaad015e24dbf1a1ad5d
(cherry picked from commit 355b2e1aacbdf508842c9350d425eba509ee5e46)