Add subordinate_units for application.Application

Author: cderici

juju/application.py

@@ -64,6 +64,11 @@ def units(self):
             if unit.application == self.name
         ]
 
+    @property
+    def subordinate_units(self):
+        """Returns the subordinate units of this application"""
+        return [u for u in self.units if u.is_subordinate]
+
     @property
     def relations(self):
         return [rel for rel in self.model.relations if rel.matches(self.name)]