Poll functionality is required for long running stacks.
When stack-create is passed with --poll argument, it will first print
stack-show output and then continously print the events in log format
until stack completes its action with success/failure.
This patch only implements poll for stack create action.
DocImpact
A new option --poll is added to stack-create.
Partial-Bug: #1420541
Change-Id: Ib7d35b66521f0ccca8544fd18fb70e04eaf98e5a
04b3880cb4