[backward-conversion] Recognize templates openwisp#100

Closes openwisp#100

.gitignore

@@ -54,8 +54,12 @@ target/
 
 # editors
 *.komodoproject
+.vscode
 
 # other
 *.DS_Store*
 .__*
 __DEV
+
+# TODO: Remove
+test.py

netjsonconfig/backends/base/backend.py

@@ -161,6 +161,36 @@ def json(self, validate=True, *args, **kwargs):
         config.update({'type': 'DeviceConfiguration'})
         return json.dumps(config, *args, **kwargs)
 
+    def distinct_native(self, templates=None):
+        """
+        :param templates: ``list`` containing **NetJSON** configuration dictionaries
+
+        returns a dict of configuration which contains configurations
+        that do not exist in given templates.
+
+        :returns: dict
+        """
+        config_to_send = {}
+        controller_config = self._merge_config({}, templates)
+        for section in controller_config:
+            device_section = self.config.get(section, None)
+
+            if device_section and isinstance(device_section, list):
+                for element in device_section:
+                    element_dict = dict(element)
+                    if element_dict not in controller_config[section]:
+                        config_to_send[section] = element_dict
+
+            if device_section and isinstance(device_section, dict):
+                for config in controller_config[section]:
+                    device_config = device_section.get(config, None)
+                    if device_config and device_config != controller_config[section][config]:
+                        if not config_to_send.get(section, None):
+                            config_to_send[section] = {}
+                        config_to_send[section][config] = device_config
+
+        return config_to_send
+
     def generate(self):
         """
         Returns a ``BytesIO`` instance representing an in-memory tar.gz archive