waiting for the OLT to go down in OLT soft reboot tests

Change-Id: Ib03f614d1c55ef3150045c3839e1db7b8d4247d6
diff --git a/tests/functional/Voltha_FailureScenarios.robot b/tests/functional/Voltha_FailureScenarios.robot
index 859e1f2..37f1418 100644
--- a/tests/functional/Voltha_FailureScenarios.robot
+++ b/tests/functional/Voltha_FailureScenarios.robot
@@ -593,6 +593,9 @@
         ...    REACHABLE    ${olt_serial_number}
         # Reboot the OLT using "voltctl device reboot" command
         Reboot Device    ${olt_device_id}
+        # Wait for the OLT to actually go down
+        Wait Until Keyword Succeeds    360s    5s    Validate OLT Device    ENABLED    UNKNOWN    UNREACHABLE
+        ...    ${olt_serial_number}
     END
     #Verify that ping fails
     FOR    ${I}    IN RANGE    0    ${num_all_onus}