Add CloudWatch agent for disk space and memory monitoring

[cliffano]

For users that don't have New Relic or similar tools, we want to provide the ability to monitor disk space and memory usage by enabling those metrics.

CloudWatch agent https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/Install-CloudWatch-Agent.html provides disk and memory metrics https://docs.aws.amazon.com/AmazonCloudWatch/latest/monitoring/metrics-collected-by-CloudWatch-agent.html, and there are some Puppet modules that seem to support the provisioning of CloudWatch monitoring.
One thing to note is that we would like to support the provisioning of CloudWatch monitoring on RHEL7 and Amazon Linux 2 on Packer AEM.

Consistent to other feature flags, please add a configuration property aws.install_cloudwatchagent to enable/disable CloudWatch agent installation.