Add python tests as a new prow job.

[spxtr]

#1342. We have two options here. We can either

1. put all of our tests in one job, or
2. try to find a logical grouping of tests and have several jobs, or
3. use bazel for everything.

Both are pretty good. In this PR I add pull-test-infra-py-test, as a step down the second path. There is some duplication in /images/, and I'd like to clean that up in a future PR if we go down this route. I'm partial to bazel, but that's a bit trickier.

[k8s-reviewable]

This change is 

[fejta]

I would prefer we use the same version system as with kubekins-e2e

[fejta]

/lgtm

[ixdy]

should we call this "prow py test"? or rename the other one from "prow go test" to just "go test?"

[spxtr]

Should rename the other to just "go test"

[ixdy]

I think I'd prefer having fewer jobs to worry about.

[ixdy]

Also the levels of indirection are starting to hurt my brain:

1. prow runs the test-infra-py-test container
2. the container runner checks out the test-infra master branch and runs bootstrap
3. bootstrap checks out test-infra at the desired merge commit and runs the job script
4. the job script actually finally runs the tests we care about (more python)

I guess the first three steps can be generalized/parameterized, which would reduce some of the cognitive overhead. I can't imagine having to set up all of these steps every time we want to start a new job.

[spxtr]

Yeah, it does feel a bit complicated. We can cut that back by moving bootstrap functionality into prow itself, if that makes sense.