Tune up report and downtime intervals for l2 agent

If the neutron server erroneously thinks than the l2 agent is down
it will fail to bind a port, which can lead to VM's spawn errors.

This patch aims to add support for agent checks flags.

Change-Id: If047c5d52d10a8ca9b1b765400bdd99a29550b1e
Signed-off-by: Emilien Macchi <emilien.macchi@enovance.com>
(cherry picked from commit 30885efc1f7ee1b8490474399014f1f5c931097f)
Merge "Tune up report and downtime intervals for l2 agent" into stable/havana