Jstenhouse/fixing cloudwatch delay

@lars2893 @tmacie @briandela 

This solves a bug in the cloudwatch_delay logic. See https://github.com/newrelic-platform/newrelic_aws_cloudwatch_plugin#cloudwatch-delay for some more info on the delay attribute.

Basically, if a collector sends in a start_time it needs to include the end_time. If a collector doesn't specify a start/end time then, the base collector will handle the offset.