aws-tests: Handle existing S3 bucket

Do not fail the test when attempting to
create an S3 bucket if the bucket already exists.
The reason being that it could have been
a re-run of the test because of a fluke so the existence
of the bucket is not a problem.
It is something that often happens so this should
improve CI.

Signed-off-by: Hugues Kamba-Mpiana <hugues.kambampiana@arm.com>