Allow to configure a value of maximum retry attempts

[jordeu]

Description

At the current implementation it is not possible to set the AWS max_attempts variable, and the default value is always use. On some scenarios with high concurrency the default value is too low and many errors arise.

At this pull request, if the environment variable AWS_MAX_ATTEMPTS is defined it will be use instead of the default value. This allows to increase the retries and then Goofys can handle this high concurrent scenarios without any problem.