Fix InstanceExists Waiter

[joaogbcravo]

Instead of checking just the status of DescribeInstances operation from aws, we should check if the reservations list have length > 0.

[joaogbcravo]

To have more context about this change:
I created a machine, then I destroyed it, and waited to not be seen anymore on my dashboard.
When I do describe_instances({instance_ids: [DESTROYED_INSTANCE_ID]}) it returns a good response (200 OK) with an empty reservation list, instead of InvalidInstanceIDNotFound.

This makes the Aws::Resources::Resource exists? method to be a "liar" method.

[trevorrowe]

Sorry for the slow merge. The fix looks good. We may be able to simplify the path expression and replace it with a pathAny matcher. Semantically it would be the same as the pathAny and pathAll matchers may not return empty lists.

This should go out with the next release.