Allow to specify range for cloudwatch checks

[rajiv-g]

Need to specify the range for cloudwatch checks.

Use Case:

1. I need to create the check that alert us quickly if there is no traffic in Lambda function for few minutes (Less than ten minutes)

Existing setup:
When period is specified, the range is calculated based on period(i.e., period * 10). It is going to fetch last 10 datapoints & range will multiples of 10 minutes as we increase period.

Purpose:
Plugin uses missing data window as period * 10. So it will take more to time to alert for no-data. It will get worse if period is high. If plugin allowed to customize the missing data window (Range), it will be useful to alert quickly on missing the latest datapoints.