integration test is unstable.

manager.integrationtest was failed.

java.lang.AssertionError: null
at org.junit.Assert.fail(Assert.java:86)
at org.junit.Assert.assertTrue(Assert.java:41)
at org.junit.Assert.assertTrue(Assert.java:52)
at org.opendaylight.vtn.manager.internal.VTNManagerIT.testIListenDataPacket(VTNManagerIT.java:4605)

This failure was caused by a conjunction of 4 bugs:

1. Illegal initial value of the count down latch.
This test removes flow entries installed by previous test.
So "ethers.size() * 2" must be used as the initial value of the count down
latch to wait for node0 flows. (VTNManagerIT.java: line 4518)

2. Too short timeout to wait for flow entries.
Current timeout value (2 seconds) may not be enough to wait for flow entries
to be installed if the build host is busy.

3. Need to ensure that the shortest path graph is updated.
This test needs to ensure that an edge from node1 to node0 is available
before sending packet.

4. A map which keeps installed SAL flow must be synchronized.
manager.integrationtest uses a mock-up of IPluginInFlowProgrammerService
in order to keep installed SAL flows. It needs to be synchronized because
SAL flows may be modified by multiple threads concurrently.