Replace SIGTERM by SIGQUIT in waitshutdown to enable connection draining

[mtparet]

What this PR does / why we need it:

SIGTERM kill nginx processus without draining connection because it is a "fast shutdown".

As waitshutdown is called in the prestop lifecycle hook, immediately when we ask for pod termination, it means we don't
wait any time for draining existing connections which will be immediately dropped.

Use SIGQUIT which waits connections to end before terminating nginx processus.

References :
http://nginx.org/en/docs/control.html
https://medium.com/codecademy-engineering/kubernetes-nginx-and-zero-downtime-in-production-2c910c6a5ed8

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation only

Which issue/s this PR fixes

How Has This Been Tested?

Start nginx

docker run -p 8080:80 nginx 

Open a telnet ongoing connection

telnet 127.0.0.1 8080

Send a SIGTERM

pkill -SIGTERM -f nginx

Observe that connection is immediately terminated and nginx process are killed.
Restart nginx container and the telnet command.
Send a SIGQUIT

pkill -SIGQUIT -f nginx

Observe that the connection is not terminated and all nginx process are not killed (only idle nginx are terminated)

Checklist:

• My change requires a change to the documentation.
• I have updated the documentation accordingly.
• I've read the CONTRIBUTION guide
• I have added tests to cover my changes.
• All new and existing tests passed.

[k8s-ci-robot]

@mtparet: This issue is currently awaiting triage.

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[k8s-ci-robot]

Hi @mtparet. Thanks for your PR.

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[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is NOT APPROVED

This pull-request has been approved by: mtparet
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[mtparet]

It seems it is already a QUIT #6928 (comment)

[mtparet]

Same issue/solution here #6928 (comment)