use deployed MR for python e2e tests

[isinyaaa]

Fixes: #380

Description

Python e2e tests will use an environment variable to MR, delegating clean-up to a separate script. The script is made environment-aware using envvars.

The CI has been updated to account for the changes, separating the Python workflows into 3 types to make it clearer.

How Has This Been Tested?

Merge criteria:

• All the commits have been signed-off (To pass the DCO check)

• [ ] The commits have meaningful messages; the author will squash them after approval or in case of manual merges will ask to merge with squash.
• [ ] Testing instructions have been added in the PR body (for PRs involving changes that are not immediately obvious).
• [ ] The developer has manually tested the changes and verified that the changes work.
• [ ] Code changes follow the kubeflow contribution guidelines.

If you have UI changes

• [ ] The developer has added tests or explained why testing cannot be added.
• [ ] Included any necessary screenshots or gifs if it was a UI change.
• [ ] Verify that UI/UX changes conform the UX guidelines for Kubeflow.

[google-oss-prow[bot]]

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