[psu_base]: Add method to retrieve number of PSUs available on the device (#133)

Author: jleveque

sonic_psu/psu_base.py

@@ -14,6 +14,15 @@
 class PsuBase(object):
     __metaclass__ = abc.ABCMeta
 
+    @abc.abstractmethod
+    def get_num_psus(self):
+        """
+        Retrieves the number of PSUs available on the device
+
+        :return: An integer, the number of PSUs available on the device
+        """
+        return 0
+
     @abc.abstractmethod
     def get_psu_status(self, index):
         """