[Identity] Make the identity loading more robust

[Michaelvll]

This is to avoid the newlines in the identity string to affect the identity comparison to make the identity checking more robust. Try to fix #2032

Tested (run the relevant ones):

• 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: bash tests/backward_comaptibility_tests.sh

[Michaelvll]

I also met the problem in #2032 for a cluster I stopped long time ago, and this PR solved it. We should get this merged.

sky down test-*                                                     
Terminating 3 clusters: test-tqa. Proceed? [Y/n]: 
'test-tqa' (GCP) is owned by account ['zhwu@berkeley.edu [project_id=skypilot-xxx]'], but the activated account is ['zhwu@berkeley.edu\n [project_id=skypilot-xxx]'].

[cblmemo]

Although I haven't encountered this issue, the code looks good to me 🫡