replace log with glog for all tests

[jessiezcc]

Fixes #1066, #763

• replace log with glog for all tests
• update document with glog instructions

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[dprotaso]

I'd like to be consistent and use the same logger within the whole project/repo. Thus not use glog but zap

These PRs is where the work was done for the controllers:
#998
#978
#933
#888

thoughts?

[jessiezcc]

@dprotaso, we are leveraging glog VLog for verbose logs, and also get underlying k8s V logs for free, does zap have similar capability?
Also chatted with Mustafa (mdemirhan@), sounds like with zap, there is also extra work for log readability, and we still need glog to get k8s outputs, so I think glog alone should satisfy our test log need for now.

[jessiezcc]

Actually, I'm opening a separate issue to track further about how test log can leverage zap structured log for better failure troubleshooting. Mustafa listed a few goodness of structured log that's beneficial for test failure diagnose. Would like to get this one in first though.

[mattmoor]

Havent cracked the diff here, but generally I share @dprotaso's sentiment. We decided to go with zap, so let's go with zap.

[jessiezcc]

I've created #1117 to track using zap in test log. My plan is to get this one merged first. Also it will take more than a week for me to get zap change in as I will be away at summit for almost a week (coming week). Let me know if you have string objections on having this PR merged, @mattmoor @dprotaso .

[adrcunha]

/lgtm

[adrcunha]

/test pull-knative-serving-integration-tests