Implement engine listener service for multi-engine support

The engine listener service provides a way for engines to confirm that
other engines are still alive.  For example, during acquisition of a
stack lock, if an existing stack lock is found the engine will attempt
to communicate with the engine who owns that lock.  If the engine
responds affirmatively, the lock will be considered active.

Blueprint multiple-engines

Change-Id: I6d8f1305953212408f19688a1e0af81c44e5e5b4
Make the mountpoint property description clearer

There is an issue where the mountpoint may not be honored
so add this note to the property documentation so users can be
aware of the issue.

Change-Id: Iddab9b1f6e5fadddd00e651e92b8b9c922a59347
Closes-bug: #1231705
Merge "Make the mountpoint property description clearer"
Merge "Implement engine listener service for multi-engine support"