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.

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[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

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[aledbf]

@mootpt thanks!

[mootpt]

sure thing