Fixes "OVS Agent doesn't start on Windows"

The OVS Agent doesn't start on Windows because of Windows
specific import.

The fix is trivial and consists in implementing InterfacePollingMinimizer
as a empty class for Windows and importing polling from common.

Closes-Bug: #1556992

Change-Id: Ie43c841bf05b4b011a74a3f8198576cbc41bcbbb

neutron/agent/common/polling.py

@@ -22,3 +22,4 @@ else:
     from neutron.agent.linux import polling
 
 get_polling_manager = polling.get_polling_manager
+InterfacePollingMinimizer = polling.InterfacePollingMinimizer

neutron/agent/windows/polling.py

@@ -22,3 +22,9 @@ from neutron.agent.common import base_polling
 def get_polling_manager(minimize_polling, ovsdb_monitor_respawn_interval):
     pm = base_polling.AlwaysPoll()
     yield pm
+
+
+# TODO(atuvenie): make this manager inherit from
+# that fully fledged polling manager interface
+class InterfacePollingMinimizer(object):
+    pass

neutron/plugins/ml2/drivers/openvswitch/agent/ovs_neutron_agent.py

@@ -34,7 +34,6 @@ from neutron.agent.common import polling
 from neutron.agent.common import utils
 from neutron.agent.l2.extensions import manager as ext_manager
 from neutron.agent.linux import ip_lib
-from neutron.agent.linux import polling as linux_polling
 from neutron.agent import rpc as agent_rpc
 from neutron.agent import securitygroups_rpc as sg_rpc
 from neutron.api.rpc.callbacks import resources
@@ -1913,7 +1912,7 @@ class OVSNeutronAgent(sg_rpc.SecurityGroupAgentRpcCallbackMixin,
                 # BasePollingManager that will be implemented by AlwaysPoll as
                 # no action and by InterfacePollingMinimizer as start/stop
                 if isinstance(
-                    polling_manager, linux_polling.InterfacePollingMinimizer):
+                    polling_manager, polling.InterfacePollingMinimizer):
                     polling_manager.stop()
                     polling_manager.start()
             elif ovs_status == constants.OVS_DEAD: