avoid excessive database calls while loading events

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