avoid excessive database calls while loading events

list_events constructs Event objects through Event.load, which causes
every event record previously loaded into memory to be reloaded again.
Furthermore, Event.load will also reload the associated stack, thus
all its resources and template. Another bad thing with that is
reloading a stack will lead to many extra stack static resolution
(including calls to nova to resovle Fn::GetAZs if it's referenced in
the template).

Fixes bug #1195793

Change-Id: I4f14c9a2a4f990b753a6e1405e27eb44c53537c9
Merge "avoid excessive database calls while loading events"