add getFullyStarted() to OpenFlowPluginProviderImpl and fix the order of isOnlineFuture vs readyRunnable: For a test to be able to await getFullyStarted() and then check diagstatus, that (diagstatus) needs to be set BEFORE, otherwise there is a race condition.

This has come up while writing the testDiagStatus() for the OpenFlowPluginWiringTest for opendaylight-simple.