Port forwarding L3 agent extension needs to clean cache when
router is DOWN, otherwise the port forwarding extension will
skip all objects processing when router is UP again, because
the cache is hitting.
This patch addresses this issue.
Change-Id: Idd65f1eda020ada9a76c39904efa8d55680d9e6b
Closes-bug: #1917393
(cherry picked from commit 0582146708)