Refactored entity object resolution.
    
    The IPA.get_entity() has been modified to accept either entity name
    or entity object. If it receives an entity object it will return
    the object itself. Otherwise, it will resolve the name in the entity
    registry.
    
    The other_entity variables have been modified to store a reference
    to the entity object instead of its name. The test cases have been
    modified to use real entity objects instead of just the names.
    
    Ticket #2042