Change guard interval for agent failure detection

[AlexeyKasatkin]

Now agent failure is reported when there was no keepalive message from the agent for more than 2*report_interval time.

Closes: https://github.com/Mirantis/k8s-netchecker-server/issues/87

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[alex-arzner-pro]

Reviewed 2 of 2 files at r1. Review status: all files reviewed at latest revision, all discussions resolved.

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[alex-arzner-pro]

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Review status: :shipit: all files reviewed at latest revision, all discussions resolved, all commit checks successful.

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