[Spot] Fix memory contraint for spot controller and backward compatibility for spot commands

[Michaelvll]

Fix spot controller memory contraint.
This is a bug introduced by #3191.

This also fixes the issue with the job_lib assertion for spot jobs. To reproduce:

1. git checkout 053d0ba2b58705c3bc5540729eb5084911e7fa9a; sky spot launch -n test-long-run --cloud gcp --cpus 2 -d "echo hi; sle ep 100000000000000"
2. sky checkout master
3. sky spot launch -n test-1 echo hi
4. sky spot logs 1 fails due to the assertion

After this PR, it should fix this backward compat issue for the following two cases:

1. if the original commit is before 053d, it should work normally with the current PR
2. if the original commit is the current master, and the spot controller is updated to the current master as the repro script above, after sky start sky-spot-controller-<hash>, all the commands should work normally.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • sky spot launch -n test echo hi without a controller
  • Reproducible code above.
• 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: bash tests/backward_comaptibility_tests.sh

[concretevitamin]

LGTM