Add alarming cron tab feature for back ups

[jennysivapalan]

Based on https://github.com/guardian/machine-images/pull/80 I have added the alarming crontab feature for mongo-opsmanager-backup.

I'm not a 100% sure I've done this right - is the best way to test this to increase the autoscaling group and check or will this potentially cause multiple back ups?

Also we need to create a PagerDuty service key.

cc @nlindblad @sihil

[niklasvincent]

Looks good! 👍 Yeah, if you scale up to two hosts you can just have a look at the crontab entry for the ubuntu user on the new one: crontab -l -u ubuntu

[sihil]

No longer use machine-images, so closing.