Fix race condition after launching EC2 instance

It is possible for an instance to not immediately exist after it is
launched. Previously, InstanceStateRefreshFunc would crash if this race
condition were realized.

This change takes the exact same approach of the function above,
AMIStateRefreshFunc, treating 'InvalidInstanceID.NotFound' as if there
were an empty result.
Merge pull request #522 from patricklucas/fix_instance_not_found_race

builder/amzon: fix race condition after launching EC2 instance