Add more explicit console logging + minor fixes

* The checks with keywords such as "Should Not Be Empty", "Should
  Be Equal As"... should have a more explicit message on fail, so that
  the console output shows a clearer failing reason.
* Other minor fixes included in the commit are:
	* The Sadis file comparison to None string.
	* The KUBECTL_CONFIG variable not properly set, altough
	  apparently unused.

Change-Id: I9305ba791dd43c9088a7bcbcc9af568ae127f60f
diff --git a/tests/functional/Voltha_PODTests.robot b/tests/functional/Voltha_PODTests.robot
index ddc687e..260fc94 100755
--- a/tests/functional/Voltha_PODTests.robot
+++ b/tests/functional/Voltha_PODTests.robot
@@ -301,7 +301,7 @@
         ${olt_device_id}=    Get OLTDeviceID From OLT List    ${olt_serial_number}
         ${rc}    ${output}=    Run and Return Rc and Output
         ...    voltctl -c ${VOLTCTL_CONFIG} device disable ${olt_device_id}
-        Should Be Equal As Integers    ${rc}    0
+        Should Be Equal As Integers    ${rc}    0   Could not disable device ${olt_device_id}
         Wait Until Keyword Succeeds    ${timeout}    5s    Validate OLT Device    DISABLED    UNKNOWN    REACHABLE
         ...    ${olt_serial_number}
     END
@@ -430,7 +430,7 @@
         ...    REACHABLE    ${src['olt']}
         ${rc}    ${output}=    Run and Return Rc and Output
         ...    voltctl -c ${VOLTCTL_CONFIG} device disable ${onu_device_id}
-        Should Be Equal As Integers    ${rc}    0
+        Should Be Equal As Integers    ${rc}    0   Could not disable device ${olt_device_id}
         Wait Until Keyword Succeeds    ${timeout}    5s
         ...    Validate Device    DISABLED    UNKNOWN
         ...    REACHABLE    ${src['onu']}    onu=false
@@ -445,7 +445,7 @@
         ${olt_device_id}=    Get OLTDeviceID From OLT List    ${olt_serial_number}
         ${rc}    ${output}=    Run and Return Rc and Output
         ...    voltctl -c ${VOLTCTL_CONFIG} device disable ${olt_device_id}
-        Should Be Equal As Integers    ${rc}    0
+        Should Be Equal As Integers    ${rc}    0   Could not disable device ${olt_device_id}
         Wait Until Keyword Succeeds    ${timeout}    5s    Validate OLT Device    DISABLED    UNKNOWN    REACHABLE
         ...    ${olt_serial_number}
     END