Fix for bug 764.

Improved logging.

  * Don't record FLOW_MOD failure if the target node does not exist.
  * Record inventory change as informational log.

Change-Id: Ic7dab1f2e1b9dcd800ab70c62c15880e5c55f7a2
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>
(cherry picked from commit 83ec37e40293a71443e45665ee2d35a40b97a2bd)