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

Change-Id: Ib03f614d1c55ef3150045c3839e1db7b8d4247d6
diff --git a/tests/dt-workflow/Voltha_DT_FailureScenarios.robot b/tests/dt-workflow/Voltha_DT_FailureScenarios.robot
index 910ff28..0e1ea73 100755
--- a/tests/dt-workflow/Voltha_DT_FailureScenarios.robot
+++ b/tests/dt-workflow/Voltha_DT_FailureScenarios.robot
@@ -453,6 +453,9 @@
     #...           AND             Delete Device and Verify
     # 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}
     #Verify that ping fails
     FOR    ${I}    IN RANGE    0    ${num_onus}
         ${src}=    Set Variable    ${hosts.src[${I}]}