Logstash_checker script for canary deploys
This patch adds a basic script that compares error rates before a deploy with
those after a deploy. To do this, it queries logstash for a given host &
service name, and calculates mean error / fatal rates before & after a deploy
time, specified via the --delay parameter. If the error ratio exceeds a
threshold (2 by default), the script returns an error.
Once integrated into the deploy process, this script should prevent badly
broken code from being deployed to production.