Reconfigure SNS notification targets for Auto Scaling progress

This last minute change addresses https://github.com/cityindex/logsearch/issues/241 in conjunction with https://github.com/cityindex/logsearch-config/issues/43.

@mrdavidlaing - the changes are trivial, but effectively untested, so I'll leave the merge decision up to you (i.e. postponing 'til the next deployment is no issue and will just retain a bit more alarm noise for a few days, eventually).