[VOL-4155] Added keyword to wait for a particular device to be recognized as an OLT in ONOS, needed in the multi-stack scale test

Change-Id: I6d2442b24d55af489b1c4847a21d82cee31cdb4a
diff --git a/tests/scale/Voltha_Scale_Tests.robot b/tests/scale/Voltha_Scale_Tests.robot
index 3537b36..902d367 100644
--- a/tests/scale/Voltha_Scale_Tests.robot
+++ b/tests/scale/Voltha_Scale_Tests.robot
@@ -103,7 +103,10 @@
 OLTs in ONOS
     [Documentation]  Check that ONOS recognize the correct number of OLTs
     [Tags]  activation  plot-onos-olts
-    Wait for Olts in ONOS   ${onos_ssh_connection}  ${olt}
+    ${onos_devices}=    Compute Device IDs
+    FOR     ${deviceId}     IN  @{onos_devices}
+        Wait for Olt in ONOS   ${onos_ssh_connection}  ${deviceId}
+    END
 
 Onu Activation in VOLTHA
     [Documentation]    Check that all ONUs reach the ACTIVE/ENABLED state in VOLTHA