Terminate orphaned instances from CreateFleet requests

[haugenj]

When using CreateFleet to provision new instances, there's a chance that instances are returned from the call but then not attached to the ASG. Instances that are not attached to the ASG will never be terminated by Esclator, so they'll stay running in the user's account indefinitely. This happens in two cases:

1. When not all instances are in a "ready" state before the instance ready timeout is hit. In this case the full number of instances become orphaned
2. When a call to AttachInstances fails. In this case any instances that haven't been attached to the instance become orphaned (happens in batches of 20)

Here's a screenshot of this happening to me for this scenario:

• 1 instance was requested but failed to be ready before the timeout
• next loop 1 instance was requested and was ready in time and attached to the ASG
• 5 instances were requested but at least one wasn't ready in time
• 5 instances were requested and successfully attached to the ASG
• When my job was finished half of the instances did not terminate

This change handles these cases and calls TerminateInstances with the list of instances that haven't been attached to the ASG. To try and mitigate the issue of these failures happening indefinitely, say if the timeout value is too low, a fatal is logged if this happens three times in a row.

[Jacobious52]

Hi @haugenj

Thanks for your PR, great to see some more contribution to the Fleet support!

Just a few changes and discussion points from me, sorry for the comment spam!

[Jacobious52]

LGTM! Great work on the new tests!