[AWS] Add security_group_name back

[Michaelvll]

The security_group_name feature was accidentally removed when we added the schema check for ~/.sky/schemas.py. This PR is to add it back and fixes user requests in #1354.

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-aws --cloud aws --cpus 2 with the following config yaml:

aws:
  security_group_name: launch-wizard-7

• 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