This is done to avoid repeated declaration of not-implemented
functions for each gateway. It also avoids a possible bug in go
https://github.com/golang/go/issues/18468 which is triggered on
our multiple PRs already.
Gateway interface implementations of GetBucketInfo() under
azure and s3 gateway did not perform any bucketname input
validation resulting in incorrect responses when the tests
are expecting InvalidBucketName.
Fixes#4983
