Use pure python entrypoints to unbreak skaffold debugging

[j-windsor]

Fixes #500.

Change summary:

• Reverts Python/Flask - Replaces gunicorn with uWSGI  #463 and bumps Gunicorn version to address the concern that prompted the switch to uwsgi.
• loadgenerator now calls locust directly instead of from a bash script. Dockerfile was modified to support debugging locust files.

Debugging will still not work until this is fixed on the skaffold side: https://github.com/GoogleContainerTools/container-debug-support/issues/83

[askmeegs]

Hi Jay, could you please provide a bit more context on why this PR is needed?
Gunicorn gave us some issues related to readiness/liveness probes and that was what prompted the move from Gunicorn to UWSGI.

#447

It does seem like the underlying bug that caused our gunicorn issues -- benoitc/gunicorn#1913 -- has been fixed, and gunicorn has put out an updated release. but I'd rather not do another backend overhaul unless there's a concrete benefit behind doing so.

[j-windsor]

Hi @askmeegs! I provided some rationale and encountered error messages in the linked issue, but the TLDR is that running skaffold debug or running debug in Cloud Code will not work with this sample. Skaffold currently just fails and exits (@briandealwis has a bug to allow it to fail silently instead: GoogleContainerTools/container-debug-support#81), and it would be much more ideal if users can actually link their debugger to this sample, and if it worked out of the box with skaffold and Cloud Code.

[kelsk]

nit: license should be 2021

[j-windsor]

I updated all touched files to have the correct copyright year.

[j-windsor]

Thanks for the review @kelsk! I addressed your comment. PTAL.

[kelsk]

@j-windsor perfect, thanks!

@askmeegs For a little more context, the main goal of configuring Bank of Anthos to be debuggable using Cloud Code/skaffold debug is to let folks use the Cloud Code debug feature without additional setup.

If it's not feasible to revert back to using gunicorn in the near future, maybe we could keep this version in a specific branch and have that branch cloned when a user opens BoA in Cloud Shell. (We could add a one-liner instruction for users to clone the specific branch if they want to try debugging on Cloud Code in their IDE.) WDYT?

[askmeegs]

Tested locally in a GKE cluster, and a cluster with Anthos Service Mesh. LGTM.

No readiness probe issues observed. (which was the gunicorn bug that caused us to move to uwsgi - ). Will need to keep an eye on this. #447

[j-windsor]

Thanks @askmeegs and @kelsk! Feel free to merge whenever.