Increase TestDhcpAgentHA.agent_down_time to 30 seconds
Currently there is a delay (around 20 seconds) between the the agent
update call and the server reply, due to the testing servers load. This
time should be higher than the agent-server communication delay but
still short enough to detect when the DHCP agent is dead during the
active wait during the DHCP agent network rescheduling.
"log_agent_heartbeats" is activated to add information about when the
server has processed the agent report status call. This log will allow
to check the different between the server updating time and the previous
agent heartbeat timestamp.
(cherry picked from commit