Use Message.existing() to create existing messages
Use Message.existing() to create existing messages

When the messages have been created by the service we can simply use the
Message.existing() function to create a message to return with
loaded=True.

Change-Id: I7c013e97b7a8ff1badf6e5a45a5b485c62a28261

stackforge/python-openstacksdk,openstack/python-openstacksdk,dudymas/python-openstacksdk,openstack/python-openstacksdk,mtougeron/python-openstacksdk,briancurtin/python-openstacksdk,dudymas/python-openstacksdk,dtroyer/python-openstacksdk,stackforge/python-openstacksdk,briancurtin/python-openstacksdk,dtroyer/python-openstacksdk,mtougeron/python-openstacksdk