Update aws_custom.sh

The objective to this script, it's monitoring custom metrics (for example memory and disk space).

It's necessary you configure before this, the AWS linux script plugins, see this page: http://docs.aws.amazon.com/AmazonCloudWatch/latest/DeveloperGuide/mon-scripts.html 

To use this script, execute: aws_custom.sh <value to warning> <value to critical> <path to AWS autentication file> <dimensions> <Metric Name> <Namespace>

Example: aws_custom.sh 50 90 /etc/sensu/conf.d/custom/awscreds.conf InstanceId=i-ab197dd2 MemoryUtilization System/Linux

Example: aws_custom.sh 50 90 /etc/sensu/conf.d/custom/awscreds.conf Filesystem=/dev/xvda1,InstanceId=i-ab137dd2,MountPath=/ MemoryUtilization System/Linux