Import mock psu object for testing LED (sonic-net#119)

Updating for completeness on how mock objects need to be imported

```
mprabhu@565bc0455e84:/sonic/src/sonic-platform-daemons/sonic-psud$ python2 setup.py test
running pytest
running egg_info
writing sonic_psud.egg-info/PKG-INFO
writing top-level names to sonic_psud.egg-info/top_level.txt
writing dependency_links to sonic_psud.egg-info/dependency_links.txt
reading manifest file 'sonic_psud.egg-info/SOURCES.txt'
writing manifest file 'sonic_psud.egg-info/SOURCES.txt'
running build_ext
==================================================================================== test session starts =====================================================================================
platform linux2 -- Python 2.7.16, pytest-3.10.1, py-1.7.0, pluggy-0.8.0
rootdir: /sonic/src/sonic-platform-daemons/sonic-psud, inifile: pytest.ini
plugins: cov-2.6.0
collected 3 items

tests/test_psud.py ...                                                                                                                                                                 [100%]

---------- coverage: platform linux2, python 2.7.16-final-0 ----------
Name           Stmts   Miss  Cover
----------------------------------
scripts/psud     355    216    39%
Coverage HTML written to dir htmlcov
Coverage XML written to file coverage.xml


================================================================================== 3 passed in 0.16 seconds ==================================================================================
```

sonic-psud/scripts/psud

@@ -20,10 +20,8 @@ try:
     # If unit testing is occurring, mock swsscommon and module_base
     if os.getenv("PSUD_UNIT_TESTING") == "1":
         from tests import mock_swsscommon as swsscommon
-        from tests.mock_device_base import DeviceBase
     else:
         from swsscommon import swsscommon
-        from sonic_platform_base.device_base import DeviceBase
 except ImportError as e:
     raise ImportError (str(e) + " - required module not found")
 
@@ -256,11 +254,14 @@ class PsuChassisInfo(logger.Logger):
     def _set_psu_master_led(self, master_status):
         try:
             try:
-                from sonic_platform.psu import Psu
+                if os.getenv("PSUD_UNIT_TESTING") == "1":
+                    from tests.mock_platform import MockPsu as Psu
+                else:
+                    from sonic_platform.psu import Psu
             except ImportError as e:
-                from tests.mock_platform import MockPsu as Psu
+                raise ImportError (str(e) + " - required module not found")
 
-            color = DeviceBase.STATUS_LED_COLOR_GREEN if master_status else DeviceBase.STATUS_LED_COLOR_RED
+            color = Psu.STATUS_LED_COLOR_GREEN if master_status else Psu.STATUS_LED_COLOR_RED
             Psu.set_status_master_led(color)
         except NotImplementedError as e:
             pass

sonic-psud/tests/mock_platform.py

@@ -1,6 +1,9 @@
-from .mock_device_base import DeviceBase
-
 class MockDevice:
+    STATUS_LED_COLOR_GREEN = "green"
+    STATUS_LED_COLOR_AMBER = "amber"
+    STATUS_LED_COLOR_RED = "red"
+    STATUS_LED_COLOR_OFF = "off"
+
     def __init__(self):
         self.name = None
         self.presence = True
@@ -24,7 +27,7 @@ def get_serial(self):
 
 class MockPsu(MockDevice):
 
-    psu_master_led_color = DeviceBase.STATUS_LED_COLOR_OFF
+    psu_master_led_color = MockDevice.STATUS_LED_COLOR_OFF
 
     def __init__(self, psu_presence, psu_status, psu_name):
         self.name = psu_name

sonic-psud/tests/test_psud.py

@@ -6,7 +6,6 @@
 from sonic_py_common import daemon_base
 
 from .mock_platform import MockChassis, MockPsu, MockFanDrawer, MockModule
-from .mock_device_base import DeviceBase
 
 SYSLOG_IDENTIFIER = 'psud_test'
 NOT_AVAILABLE = 'N/A'
@@ -148,7 +147,7 @@ def test_psuchassis_check_power_budget():
 
     assert float(fvs[CHASSIS_INFO_TOTAL_POWER_SUPPLIED_FIELD]) < float(fvs[CHASSIS_INFO_TOTAL_POWER_CONSUMED_FIELD])
     assert chassis_info.master_status_good == False
-    assert MockPsu.get_status_master_led() == DeviceBase.STATUS_LED_COLOR_RED
+    assert MockPsu.get_status_master_led() == MockPsu.STATUS_LED_COLOR_RED
 
     #Add a PSU
     psu = MockPsu(True, True, "PSU 2")
@@ -164,4 +163,4 @@ def test_psuchassis_check_power_budget():
 
     assert float(fvs[CHASSIS_INFO_TOTAL_POWER_SUPPLIED_FIELD]) > float(fvs[CHASSIS_INFO_TOTAL_POWER_CONSUMED_FIELD])
     assert chassis_info.master_status_good == True
-    assert MockPsu.get_status_master_led() == DeviceBase.STATUS_LED_COLOR_GREEN
+    assert MockPsu.get_status_master_led() == MockPsu.STATUS_LED_COLOR_GREEN