Expose periodSeconds and failureThreshold as configuration in nais.yaml

[davidsteinsland]

This will enable faster deploys, as we can use a smaller value for initialDelay and a higher value for failureThreshold instead.

The default values er fetched from Kubernetes: https://kubernetes.io/docs/api-reference/v1.8/#probe-v1-core

[jhrv]

👌 maybe also add this to nais_example.yaml with a description of what it does