StackResource add show_deleted option to nested() accessor method

Adds the option to return a stack object even if the stack has been
deleted, which is slightly more convenient when polling for DELETE
COMPLETE state, as you no longer have to handle NotFound when the
delete happens.

Change-Id: I9074bfcec84836cc789cf9f5ecd0a904f020ccb9
blueprint: decouple-nested
Merge "StackResource add show_deleted option to nested() accessor method"