AWS deprovision: periodically re-terminate shutting-down-for-a-while instances

We had a dozen or so instances in the CI account that were stuck in shutting-down for days. eparis@redhat.com opened an AWS ticket, but they didn't really investigate. Requesting a fresh termination removed the instances. Word from AWS folks is that being stuck for 15m or more is a sign of trouble, so logging a warning and re-terminating any instances at least that often seems reasonable.

Sometimes re-terminating helps, and sometimes it doesn't (or maybe that didn't quite get as far as re-terminating?), but asking for a fresh termination every 15m or so doesn't seem like it would have negative consequences. Log line should definitely whine about AWS not terminating ("consider filing a ticket with AWS support").