[Metricbeat] Create elb metricset using light weight module

[kaiyan-sheng]

This PR is to use #12465 to create elb metricset for aws. elb metricset collects metrics from aws cloudwatch with namespace AWS/ELB using aws module cloudwatch metricset.

There is also a pre defined dashboard added in this PR:

closes #11701

How to test this:

1. Make sure the AWS environment you are using for testing has ELB, which is a part of EC2. So to check if your environment has load balancers, go to EC2 service and then click Load Balancers on the left side: https://console.aws.amazon.com/ec2/v2/home?region=us-east-1#LoadBalancers:
2. Make sure AWS credentials are correct and valid in ~/.aws/credentials under profile test-mb
3. Download Metricbeat and enable aws module: ./metricbeat modules enable aws
4. Change aws.yml to enable elb metricset:

- module: aws
  period: 300s
  credential_profile_name: test-mb
  metricsets:
    - elb

5. Check Kibana dashboard or Discover for elb data

[jsoriano]

This is looking good, are you planning to create a dashboard? 🙂

[kaiyan-sheng]

This is looking good, are you planning to create a dashboard? 🙂

@jsoriano Yes, after adding a dashboard I will make this PR in review (officially 😬)!

[kaiyan-sheng]

@jsoriano I added a dashboard for ELB and moved this PR to in review. I waited for a while to try to get better looking data but so far no luck by using the test aws env 😬

[jsoriano]

Nice dashboard! Just one detail.