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

Change-Id: Ib03f614d1c55ef3150045c3839e1db7b8d4247d6
diff --git a/tests/tt-workflow/Voltha_TT_FailureScenarios.robot b/tests/tt-workflow/Voltha_TT_FailureScenarios.robot
index 8cef76e..02a56a0 100755
--- a/tests/tt-workflow/Voltha_TT_FailureScenarios.robot
+++ b/tests/tt-workflow/Voltha_TT_FailureScenarios.robot
@@ -237,6 +237,9 @@
         ${olt_device_id}=    Get OLTDeviceID From OLT List    ${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
     FOR    ${I}    IN RANGE    0    ${num_all_onus}
         ${src}=    Set Variable    ${hosts.src[${I}]}