Building ONU list per OLT

Change-Id: I9b56b5d7df2601fabe90652f6b15c7bf789334a2
diff --git a/libraries/voltctl.robot b/libraries/voltctl.robot
index c7819e1..59b85f9 100755
--- a/libraries/voltctl.robot
+++ b/libraries/voltctl.robot
@@ -674,10 +674,11 @@
     [Return]    ${id}
 
 Build ONU SN List
-    [Arguments]    ${serial_numbers}    ${num_onus}=${num_all_onus}
-    [Documentation]    Appends all ONU SNs to the ${serial_numbers} list
+    [Arguments]    ${serial_numbers}    ${olt_serial_number}=${EMPTY}    ${num_onus}=${num_all_onus}
+    [Documentation]    Appends all ONU SNs for the given OLT to the ${serial_numbers} list
     FOR    ${INDEX}    IN RANGE    0    ${num_onus}
-        Append To List    ${serial_numbers}    ${hosts.src[${INDEX}].onu}
+    Run Keyword IF  "${olt_serial_number}"=="${hosts.src[${INDEX}].olt}" or "${olt_serial_number}"=="${EMPTY}"
+    ...   Append To List    ${serial_numbers}    ${hosts.src[${INDEX}].onu}
     END
 
 Get SN From Device ID