Fix - Load AWS profile from config if not provided as CLI arg

[danielxia-sprout]

Proposed fix for #180, #155

profile comes from an optional command line argument --profile and could be None. AwsAuth is fine with profile=None being passed into the constructor in which case it falls back to the AWS profile from the configuration. However, that fallback within AwsAuth cannot change the value of profile, which are then incorrectly treated as the effective profile instead of a command line argument.

Testing

For #180, with the patch, it recognizes the profile from the configuration, instead of throwing a stack trace.

$  okta-awscli --okta-profile <some-profile> -v --debug 
DEBUG - Setting AWS role to ...
DEBUG - Setting AWS partition to AwsPartition.AWS
DEBUG - Setting AWS profile to production-apps
DEBUG - Checking STS token against ARN partition: AwsPartition.AWS
INFO - STS credentials are valid. Nothing to do.

For #155, it rotates the credentials when it is no longer valid

$ okta-awscli --okta-profile <some-profile> -v --debug 
DEBUG - Setting AWS role to ...
DEBUG - Setting AWS partition to AwsPartition.AWS
DEBUG - Setting AWS profile to production-apps
DEBUG - Checking STS token against ARN partition: AwsPartition.AWS
INFO - Temporary credentials have expired. Requesting new credentials.
DEBUG - Setting MFA factor to OKTA
INFO - App Link set as: ...