Add external_instance_event() method to compute manager

This would allow external services to send notifications in to
the compute manager, which could have one or more threads waiting
to be unblocked. It would also provide a way for us to process
generic events like "update your instance cache for $instance".

Related to blueprint admin-event-callback-api

Change-Id: If87cc7fdb1ebcfa2a1d33f5e864c43ed0290beb6
Merge "Add external_instance_event() method to compute manager"