messaging: Provide metadata for messages

[vinzenz]

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]

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