Stacker shouldn't exit until UPDATE is actually complete.

[maraca]

When updating the stack, Stacker will return if the status shows something different than UPDATE_IN_PROGRESS. It should only return when one of the following happens CREATE_COMPLETE UPDATE_COMPLETE UPDATE_ROLLBACK_COMPLETE

Cronbox Status => UPDATE_IN_PROGRESS
Cronbox Status => UPDATE_IN_PROGRESS
...
Cronbox Status => UPDATE_IN_PROGRESS
Cronbox Status => UPDATE_IN_PROGRESS
Cronbox Status => UPDATE_COMPLETE_CLEANUP_IN_PROGRESS