Test PyPI pre-releases but allow for failures

[consideRatio]

Using python setup.py install will install PyPI pre-releases of
packages while pip install -r requirements.txt won't do that. This
lead to CI tests failing as reported in #421. This commit updates the CI
build setup to run two parallell tests, one where we don't use
pre-releases and one where we do but allow them to fail and won't wait
on the run to complete either.

Fixes #421

Special notes for your reviewer:

• Closes Work around k8s client issue to avoid blocking CI tests #422 by providing an alternative approach. The difference was described here: Work around k8s client issue to avoid blocking CI tests #422 (comment)
• I've come across this issue in another project as well where this approach was adopted for a ReadTheDocs build, for a trace to that see Documentation Missing jupyterhub/kubespawner#370 (comment).

Release note:

NONE

---

This change is 

[k8s-ci-robot]

Hi @consideRatio. Thanks for your PR.

I'm waiting for a kubeflow member to verify that this patch is reasonable to test. If it is, they should reply with /ok-to-test on its own line. Until that is done, I will not automatically test new commits in this PR, but the usual testing commands by org members will still work. Regular contributors should join the org to skip this step.

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[abhi-g]

/ok-to-test

[abhi-g]

/test kubeflow-presubmit

[abhi-g]

/test kubeflow-fairing-presubmit

[abhi-g]

/lgtm
/approve

[k8s-ci-robot]

[APPROVALNOTIFIER] This PR is APPROVED

This pull-request has been approved by: abhi-g

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[jinchihe]

@consideRatio Great thanks!