Document when to modify ELB idle timeouts and set default value to 60s

[mootpt]

What this PR does / why we need it:
This PR changes the default ELB idle timeout in the manifest to 60s. This is to allow connections to be properly closed by the ELB. By default, the NGINX keepalive_timeout is set to 75s and with the existing config of 3600s the ELB attempts to reuse closed connections.

Here is a link to the official AWS documentation that covers idle timeouts.

Special notes for your reviewer:

Slack thread: https://kubernetes.slack.com/archives/CANQGM8BA/p1536105979000100

The example in the Helm chart will also likely need to be updated

Please feel free to amend as necessary.

[k8s-ci-robot]

Thanks for your pull request. Before we can look at your pull request, you'll need to sign a Contributor License Agreement (CLA).

📝 Please follow instructions at https://git.k8s.io/community/CLA.md#the-contributor-license-agreement to sign the CLA.

It may take a couple minutes for the CLA signature to be fully registered; after that, please reply here with a new comment and we'll verify. Thanks.

---

• If you've already signed a CLA, it's possible we don't have your GitHub username or you're using a different email address. Check your existing CLA data and verify that your email is set on your git commits.
• If you signed the CLA as a corporation, please sign in with your organization's credentials at https://identity.linuxfoundation.org/projects/cncf to be authorized.
• If you have done the above and are still having issues with the CLA being reported as unsigned, please email the CNCF helpdesk: helpdesk@rt.linuxfoundation.org

Instructions for interacting with me using PR comments are available here. If you have questions or suggestions related to my behavior, please file an issue against the kubernetes/test-infra repository. I understand the commands that are listed here.

[mootpt]

/check-cla

[mootpt]

/assign @aledbf

[aledbf]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: aledbf, mootpt

The full list of commands accepted by this bot can be found here.

The pull request process is described here

Needs approval from an approver in each of these files:

• OWNERS [aledbf]

Approvers can indicate their approval by writing /approve in a comment
Approvers can cancel approval by writing /approve cancel in a comment

[aledbf]

@mootpt thanks!

[mootpt]

sure thing