Change added USER_HASH in models name to full digits (8) instead of half (4) to prevent potential hash conflict

[andylizf]

Resolves #4143

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
• All smoke tests: pytest tests/test_smoke.py
• Relevant individual smoke tests: pytest tests/test_smoke.py::test_fill_in_the_name
• Backward compatibility tests: conda deactivate; bash -i tests/backward_compatibility_tests.sh

[andylizf]

After a thorough investigation, I found changing cluster_name_on_cloud in _provision changes the handle and cluster config yaml, which further affects sky launch/start for restarting the existing clusters (INIT/STOP) with already generated cluster_name_on_cloud in the existing cluster config yaml.

Luckily, I found that #1235 had already solved this backward compatibility issue by restoring the old cluster_name_on_cloud before generating a new cluster_name_on_cloud to overwrite it. Thus, we can promise that the leakage never happens by losing the old closter_name_on_cloud.

[andylizf]

@cblmemo PTAL, thanks!

[cblmemo]

Thanks for fixing this @andylizf ! This looks good to me. cc @Michaelvll for a look here to make sure every thing looks good to you

[cblmemo]

is this constant still in use after this deletion? if not we can remove it.

[Michaelvll]

This looks good! Thanks @andylizf! Can we see if there is any manual backward compatibility tests other than the script to make sure this does not break any existing cluster, jobs, or services?

[Michaelvll]

minor: we can further increase the character space for the user hash from hex to base36, i.e. update the common_utils.generate_user_hash to further encode the user_hash with common_utils.base36_encode.