Use the AWS SDK wait until rather than our own version

The SDK now supports waiting for status changes on some resources,
including instances. So swap out our implementation for the build-in
one.
Fix error message appearing in successful test runs

Rather than returning nil the function raises an exception, so check for
that expection to determine whether the group no longer exists.
Merge branch 'use-native-wait-until'

* use-native-wait-until:
  Fix error message appearing in successful test runs
  Use the AWS SDK wait until rather than our own version

closes #29