Preconditions fails for Openflow node in ProviderNetworkManagerImpl.getProvider

The current PreCondition check in getProvider() method assumes that the node
is an OVS node. But this method is called for Openflow Node as well.
Hence the precondition fails.

java.lang.IllegalArgumentException: null
at com.google.common.base.Preconditions.checkArgument(Preconditions.java:76) ~[na:na]
at org.opendaylight.ovsdb.openstack.netvirt.impl.ProviderNetworkManagerImpl.getProvider(ProviderNetworkManagerImpl.java:40) ~[na:na]
at org.opendaylight.ovsdb.openstack.netvirt.SouthboundHandler.notifyNode(SouthboundHandler.java:366) ~[na:na]
at org.opendaylight.controller.switchmanager.internal.SwitchManager.notifyNode(SwitchManager.java:1827) ~[na:na]
at org.opendaylight.controller.switchmanager.internal.SwitchManager.addNode(SwitchManager.java:1049) ~[na:na]
at org.opendaylight.controller.switchmanager.internal.SwitchManager.updateNode(SwitchManager.java:1150) ~[na:na]
at org.opendaylight.controller.sal.implementation.internal.Inventory.updateNode(Inventory.java:115) ~[na:na]
at org.opendaylight.controller.sal.compatibility.InventoryAndReadAdapter.publishNodeUpdate(InventoryAndReadAdapter.java:728) ~[na:na]
at org.opendaylight.controller.sal.compatibility.InventoryAndReadAdapter.onNodeUpdated(InventoryAndReadAdapter.java:463) ~[na:na]
at org.opendaylight.controller.sal.compatibility.InventoryAndReadAdapter$$Broker$ListenerInvoker.onNotification(InventoryAndReadAdapter$$Broker$ListenerInvoker.java) ~[na:na]
at org.opendaylight.controller.sal.binding.impl.AbstractNotificationListenerRegistration.notify(AbstractNotificationListenerRegistration.java:38) ~[na:na]
at org.opendaylight.controller.sal.binding.impl.NotifyTask.run(NotifyTask.java:42) ~[na:na]
at java.util.concurrent.Executors$RunnableAdapter.call(Executors.java:471) [na:1.7.0_40]
at java.util.concurrent.FutureTask.run(FutureTask.java:262) [na:1.7.0_40]
at java.util.concurrent.ThreadPoolExecutor.runWorker(ThreadPoolExecutor.java:1145) [na:1.7.0_40]
at java.util.concurrent.ThreadPoolExecutor$Worker.run(ThreadPoolExecutor.java:615) [na:1.7.0_40]
at java.lang.Thread.run(Thread.java:724) [na:1.7.0_40]