Wait for ovsdb_monitor to be active before use it

There is a race between ovsdb_monitor becoming active and using
ovsdb_monitor. Sometimes, code [1] will be hit at ovs-agent startup.

The fix here will block the start of ovsdb_monitor, so that the
following code to use the ovsdb_monitor will have ovsdb_monitor be
active.

[1] https://goo.gl/RJX4I5
Closes-bug: #1584647

Change-Id: I893a3b250339006f50aa003686fb95d7f2465edc

neutron/agent/linux/polling.py

@@ -54,7 +54,7 @@ class InterfacePollingMinimizer(base_polling.BasePollingManager):
             respawn_interval=ovsdb_monitor_respawn_interval)
 
     def start(self):
-        self._monitor.start()
+        self._monitor.start(block=True)
 
     def stop(self):
         try:

neutron/tests/unit/agent/linux/test_polling.py

@@ -45,7 +45,7 @@ class TestInterfacePollingMinimizer(base.BaseTestCase):
     def test_start_calls_monitor_start(self):
         with mock.patch.object(self.pm._monitor, 'start') as mock_start:
             self.pm.start()
-        mock_start.assert_called_with()
+        mock_start.assert_called_with(block=True)
 
     def test_stop_calls_monitor_stop(self):
         with mock.patch.object(self.pm._monitor, 'stop') as mock_stop:

neutron/tests/unit/plugins/ml2/drivers/openvswitch/agent/test_ovs_neutron_agent.py

@@ -1795,6 +1795,8 @@ class TestOvsNeutronAgent(object):
 
         with mock.patch.object(async_process.AsyncProcess, "_spawn"),\
                 mock.patch.object(async_process.AsyncProcess, "start"),\
+                mock.patch.object(async_process.AsyncProcess,
+                                  "is_active", return_value=True),\
                 mock.patch.object(async_process.AsyncProcess, "stop"),\
                 mock.patch.object(log.KeywordArgumentAdapter,
                                   'exception') as log_exception,\
@@ -1877,6 +1879,8 @@ class TestOvsNeutronAgent(object):
     def test_rpc_loop_fail_to_process_network_ports_keep_flows(self):
         with mock.patch.object(async_process.AsyncProcess, "_spawn"),\
                 mock.patch.object(async_process.AsyncProcess, "start"),\
+                mock.patch.object(async_process.AsyncProcess,
+                                  "is_active", return_value=True),\
                 mock.patch.object(async_process.AsyncProcess, "stop"),\
                 mock.patch.object(
                     self.mod_agent.OVSNeutronAgent,