Create script to "vendor" test-infra/scripts

[coryrc]

Tracks the contents of test-infra/scripts into each repo at
scripts/test-infra directory.

The script provides both installation assistance (with --first-time
command-line argument) as well as its usual function of updating
scripts/test-infra directory and is intended to be called by
hack/update-deps.sh in each repo. The --branch option lets you choose
a particular branch (instead of master) if desired.

Helps with #1823 (because knative/client is preventing rollout of new image through every release and needs script updates)

[coryrc]

/assign @chizhg
/assign @chaodaiG
/assign @mattmoor
/assign @vaikas

[mattmoor]

I'm not comfortable making a change of this magnitude less than a week before we are supposed to cut the test-infra/pkg release and pinning them everywhere. This should wait until the first week of 0.16.

TBH, I haven't heard a particularly clear articulation of what the problem is, so can we start with an issue that writes up the issue in detail before we prescribe solutions?

[chizhg]

/lgtm
/hold
Feel free to unhold if you don't want to make changes for my comment.

[coryrc]

/unhold

[chizhg]

/lgtm
/approve

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