Improvements to the guestbook README, switch to .yaml files

[amygdala]

Improvements to the guestbook example README, + addressed a number of friction issues.
Addresses: #9520, #9421, #8859.
Switched from .json to .yaml, updated the kubectl output listings to be consistent with 0.19.
Also made some changes to help towards #9107 ("Guestbook Key Examples Needs To Work With Core Kubernetes"), though we still need more here.

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[k8s-bot]

GCE e2e build/test passed for commit 7aaeca9a0525f31ff829c2940e079a3b43b1ad4d.

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[amygdala]

Re: the CLA -- I added my google email to my GH profile. With that email, I'm indeed listed as being covered by the Google Corporate CLA. However, I did this commit using a different (personal) email address. I'm basically set, but not sure how to make the bot happy. In future I'll use the google addr for commits. (this is amyu@)

[amygdala]

Since this is just example code, I assume the Shippable failure is some artifact of its setup?

[k8s-bot]

GCE e2e build/test passed for commit c0806b827d03903d493a07c609314ea966c081f3.

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[caesarxuchao]

Please keep this line. I think Shippable is complaining about missing this.

[amygdala]

I re-added it, but unfortunately, Shippable still seems to be complaining.

[k8s-bot]

GCE e2e build/test passed for commit 9fcc26cd6a8d8cb4da59ea673421572312bc4a31.

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[k8s-bot]

GCE e2e build/test passed for commit 52b97859c500f447529477400b9a1206e9f79e5c.

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[k8s-bot]

GCE e2e build/test passed for commit 1996ae37d271ba5926e1a911259a036f9cb2864e.

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[amygdala]

Thank you, @caesarxuchao. I've addressed everything aside from one question related to the type: LoadBalancer explanation. We are going to need to re-flip the cla label again when we're all set (I believe it's doing this b/c my initial PR submit was not with my google email address).

[k8s-bot]

GCE e2e build/test passed for commit a9e53928fa4cce2fdb93a87760685dd18e4d2458.

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[amygdala]

@caesarxuchao okay, done (& re-squashed). I made that last yaml listing change in the README, as well as the other wording change you suggested.

[k8s-bot]

GCE e2e build/test passed for commit 22f1b07.

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[caesarxuchao]

Thank you @amygdala. Could you fix the failed tests as suggested by Shippable? I think the problem is that you change the .json files to .yaml, but the tests are still looking for .json.

[amygdala]

@caesarxuchao I don't know how to do that, actually. (clicking on the 'details' link for the failed build just redirects to https://app.shippable.com/ for some reason). Do you know what needs to be changed?

[caesarxuchao]

Does the website ask you to sign in? You should be able to sign in with your github account.