Add a way to watch a scheduler-driven update until it's complete

It would be nice for scripts that coordinate updates to have the ability to watch a scheduler-driven update until it has completed. This is currently possible by scripting the client, e.g. https://github.com/apache/aurora/blob/master/src/test/sh/org/apache/aurora/e2e/test_end_to_end.sh#L149-L166, however given this is likely a common requirement, we should encapsulate this functionality within the client itself.

Two suggestions are adding aurora update watch ... or aurora update status --watch ....

The command should periodically poll the scheduler for the status of the update and exit 0 for successful updates and 1 for failed updates.