Fixed unstable tests.

  * VTNManagerImplDisableNodesTest.testDisabledNodeAfterTimeout():
    Need more time to start PACKET_IN service on a new node.
  * VTNManagerImplClusterTest:
    Test should not fail even if cluster event is removed by timer thread.

Additional changes:

  * VTNManagerImpl: Use putEvent() to catch unexpected exception.

Change-Id: I2343417d994920ddce911d7e460fb436d7914b2e
Signed-off-by: Shigeru Yasuda <s-yasuda@da.jp.nec.com>