Implement Liveness Probe

[cmrust]

An issue on the fluent-plugin-papertrail repository exposed the need for implementing Liveness Probes here: solarwinds/fluent-plugin-papertrail#27

We've already been using the Liveness Probes referenced in that thread, internally, for our Fluentd Kubernetes DaemonSet manifests for a few months with success.

We should expose this snippet here in the fluentd-deployment README as a recommendation to users who find their fluentd processes frozen up over time.

[leecalcote]

Agreed, @cmrust.