changing time format of check_awsiops

even though the API docs say time format is fine as a string in date/time format (http://docs.aws.amazon.com/aws-sdk-php/latest/class-Aws.CloudWatch.CloudWatchClient.html#_getMetricStatistics), it's not working. This PR changes the format to a timestamp instead.

Sample output running modified version:
AWS IOPS OKAY; | VolumeReadBytes=1400832.0;;;VolumeWriteBytes=3416064.0;;;VolumeReadOps=121.0;;;VolumeWriteOps=281.0