validate_credentials needs further consideration

[venkatBala]

• Validating credentials when an instance of AWSExecutor is created results in a few problems when interacting with covalent self hosted
• If the AWS_PROFILE env variable is set, the client side configuration file gets rendered with profile=<AWS_PROFILE> value. When a user then dispatches a workflow to self-hosted the executor will get populated with the profile information parsed from the configuration file which eventually will cause an error on the self-hosted side as there will be no values for the profile.
• A fix would be to not run credential validation on executor instantiation and rather let boto3 session calls fail downstream if the credentials are expired