Add comments to iptables rules generated by node-cache app

[2rs2ts]

Comments are saved as vars and reused for similar rules. It's less important that each rule is a thorough explanation of what's going on, and more important that network administrators know where the rule came from (i.e., this app) and why it's being added.

Of course, I'm completely open to feedback on what the actual comments should say, whether there should be more unique comments rather than just the 2 I started with, how I should format the code, how to make it localizable, etc.

I tested this in my own Kubernetes cluster by confirming the result with iptables-save after rolling it out.

Fixes #566

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[dpasiukevich]

/ok-to-test

[dpasiukevich]

/lgtm
/approve

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