messaging: Provide metadata for messages

This patch introduces the necessities to provide message metadata.

The meta data can be useful to identify the time, phase, actor, topic
or hostname.

Example usage from an actor:
----------------------------
```
from leapp.libraries.stdlib import api
from leapp.models import MyModel

def needs_metadata():
    for model in api.consume(MyModel):
        md = model.message_metadata()
        if md and md.phase == 'facts':
            api.current_logger().info('Facts phase run at %s', md.timestamp)
```

Signed-off-by: Vinzenz Feenstra <[email protected]>

leapp/messaging/__init__.py

@@ -189,4 +189,4 @@ def consume(self, actor, *types):
         if types:
             filtered = set(requested.__name__ for requested in types)
             messages = [message for message in messages if message['type'] in filtered]
-        return (lookup[message['type']].create(json.loads(message['message']['data'])) for message in messages)
+        return (lookup[msg['type']].create(msg, json.loads(msg['message']['data'])) for msg in messages)

leapp/models/__init__.py

@@ -71,14 +71,50 @@ def __init__(cls, name, bases, attrs):
         super(ModelMeta, cls).__init__(name, bases, attrs)
 
 
+class MetaData(object):
+    def __init__(self, data):
+        self._data = data
+
+    @property
+    def row_id(self):
+        """ Returns the row id of the message in the message table.  """
+        return self._data.get('id')
+
+    @property
+    def actor(self):
+        """ Returns the name of the actor that has sent this message.  """
+        return self._data.get('actor')
+
+    @property
+    def phase(self):
+        """ Returns the name of the phase in which this message has been sent.  """
+        return self._data.get('phase')
+
+    @property
+    def timestamp(self):
+        """ Returns the timestamp string in ISO formst from when this message has been sent.  """
+        return self._data.get('stamp')
+
+    @property
+    def topic(self):
+        """ Returns the name of the topic of this message. """
+        return self._data.get('topic')
+
+    @property
+    def hostname(self):
+        """ Returns the name of the host (FQDN) from where this message originates. """
+        return self._data.get('hostname')
+
+
 class Model(with_metaclass(ModelMeta)):
     """
     Model is a base class for all models.
 
     Models are defining the data structure of the payload of messages and the
     metadata required, such as a name and topic.
     """
-    def __init__(self, init_method='from_initialization', **kwargs):
+    def __init__(self, metadata=None, init_method='from_initialization', **kwargs):
+        self._metadata = metadata
         super(Model, self).__init__()
         defined_fields = type(self).fields
         for key in kwargs.keys():
@@ -101,16 +137,25 @@ def __init__(self, init_method='from_initialization', **kwargs):
     Note: Dynamically added fields are ignored by the framework.
     """
 
+    def message_metadata(self):
+        """
+        Provides the message meta data of a message.
+
+        :returns: An instance of :py:class:`leapp.models.MetaData` or None
+        :rtype: :py:class:`leapp.models.MetaData` or `NoneType`
+        """
+        return self._metadata
+
     @classmethod
-    def create(cls, data):
+    def create(cls, metadata, data):
         """
         Create an instance of this class and use the data to initialize the fields within.
 
         :param data: Data to initialize the Model from deserialized data
         :type data: dict
         :return: Instance of this class
         """
-        return cls(init_method='to_model', **data)
+        return cls(metadata=MetaData(metadata), init_method='to_model', **data)
 
     def dump(self):
         """