[AWS] Adding aws assume role option, and env var detection

[clayrosenthal]

Adding an option for an AWS identity that is an assumed role, set by using the source_profile and role_arn options in your ~/.aws/config. Also refactored the identity option check to a for loop as it was getting repetitive. Also made it so the env var identity is recognized by sky check

Tested (run the relevant ones):

• Code formatting: bash format.sh
• Any manual or new tests for this PR (please specify below)
  • Testing with env vars:

sky check aws     
Checking credentials to enable clouds for SkyPilot.
  AWS: enabled                              
    Hint: AWS env is set. It will work if you use AWS only, but will cause problems if you want to use multiple clouds. To set up static credentials, try: aws configure

• Testing with assumed role:

sky check aws
Checking credentials to enable clouds for SkyPilot.
  AWS: enabled                              
    Hint: AWS assume-role is set. It will work if you use AWS only, but will cause problems if you want to use multiple clouds. To set up static credentials, try: aws configure

• Testing with custom credential process:

sky check aws                                                                            
Checking credentials to enable clouds for SkyPilot.
  AWS: enabled                              
    Hint: AWS custom-process is set. It will work if you use AWS only, but will cause problems if you want to use multiple clouds. To set up static credentials, try: aws configure

• 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

[romilbhardwaj]

Thanks @clayrosenthal!