Optionally add message envelope to notifications.

When message envelopes were added for rpc messages, notifications were
explicitly left out.  This was due to the fact that notifications are
consumed outside of OpenStack.  However, notification consumers will
likely want to eventually start taking advantage of some of the features
that the envelope is a prerequisite for, such as message signing.

This patch adds a new notification driver that produces message based
notifications including the message envelope.

Related to blueprint version-rpc-messages.

Change-Id: Iaa8b437e6b4f64053f4b02bf486516f60bb020e9
Merge "Optionally add message envelope to notifications."