[Spot] Fix the handling for spot_recovery: none

[Michaelvll]

Previously, when spot_recovery: none is provided in the task.yaml, sky launch task.yaml will raise the following error, which is unexpected:

Task from YAML spec: test.yaml
ValueError: Spot recovery strategy NONE is not supported. The strategy should be among ['FAILOVER']

This PR, makes it go through the check when the spot_recovery: none is provided.

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • sky launch -c test task.yaml with spot_recovery: none provided.
  • sky spot launch --spot-recovery none task.yaml should automatically set the spot_recovery to the default one.
• 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