add repr for event objects

When an event object gets dumped in a stack trace, because there
is no __repr__ of it, we don't get any details about what it was.
Add a __repr__ to all the event objects so that we can get this
information on a fail.

Change-Id: I9a62a4348871b748c53b7354ad1b821fe987127d
Fixes internal server error for add/remove tenant flavor access request

Fixes the internal server error (500) response when an add or remove tenant access
to a flavor is made without specifying the tenant. Note that only the V2 API code
change is required because the V3 API version is protected by the input validation.

Change-Id: I7c126920016c7004fa55efb89747e97838d62f4e
Closes-Bug: 1253014
Merge "Fixes internal server error for add/remove tenant flavor access request"
Merge "add repr for event objects"