Honor SCALE_PERIOD_MINS environment variable

[Korrigan]

Description of changes:

The lambda updates Cloudwatch alarms when scaling the stream, but does not honor the provided SCALE_PERIOD_MINS environment variable to configure alarm period (it has a hardcoded value of 5mn in updateAlarm). I modified the updateAlarm function to accept a periodMins parameter, which is fetched from the SCALE_PERIOD_MINS environment variable with a fallback to 5mn already.

By submitting this pull request, I confirm that you can use, modify, copy, and redistribute this contribution, under the terms of your choice.