deprecation: add fine-grained deprecation on models

Add fine grained deprecation, alloowing the developer to specify
individual fields of a model that are deprecated. For example:

    class A():
      deprecated = fields.Boolean()
      a = fields.Boolean()

    @deprecated('b.deprecated')
    class B():
      b = fields.Model(A)
      c = fields.Int()

Accessing (set or get) b_instance.b.deprecated triggers deprecation
warnining.

leapp/models/__init__.py

@@ -89,6 +89,26 @@ def __init__(self, init_method='from_initialization', **kwargs):
         for field in defined_fields:  # noqa; pylint: disable=consider-using-dict-items
             getattr(defined_fields[field], init_method)(kwargs, field, self)
 
+        # The instance is prepared (fields are recursively populated). Modify the instance so that if we are accessing
+        # a deprecated field we will be triggering deprecation warnings
+        if hasattr(self, '_deprecated_nodes'):
+            # depr_path is of the form ['field1', 'field2', 'field3', ..]. Traverse the path to obtain
+            # x = self.field1.field2...field{N-1}. Add field{N} to x's _deprecation_attributes which
+            # will be checked when accessing its attributes using __getattribute__
+            for depr_path in self._deprecated_nodes:
+                root = self
+                depr_path = list(depr_path)
+
+                while len(depr_path) > 1:
+                    current_field = depr_path.pop(0)
+                    root = getattr(root, current_field)
+
+                # Path has length 1
+                if getattr(root, '_deprecated_attributes', None) is None:
+                    root._deprecated_attributes = []
+
+                root._deprecated_attributes.append(depr_path[0])
+
     topic = None
     """
     `topic` has to be set to a subclass of :py:class:`leapp.topics.Topic`
@@ -113,6 +133,24 @@ def create(cls, data):
         """
         return cls(init_method='to_model', **data)
 
+    def __getattribute__(self, name):
+        try:
+            depr_attribs = super().__getattribute__('_deprecated_attributes')
+            if name in depr_attribs:
+                raise NotImplementedError('Register a Deprecation warning here.')
+            return  super().__getattribute__(name)
+        except AttributeError:  # There are no _deprecated_attributes, access the fields normally
+            return super().__getattribute__(name)
+
+    def __setattr__(self, name, value):
+        try:
+            depr_attribs = super().__getattribute__('_deprecated_attributes')
+            if name in depr_attribs:
+                raise NotImplementedError('Register a Deprecation warning here.')
+            return super().__setattr__(name, value)
+        except AttributeError:
+            return super().__setattr__(name, value)
+
     def dump(self):
         """
         Dumps the data in the dictionary form that is safe to serialize to JSON.