Manta has the ability to allow users to authenticate with a
username other than the main account. We want to expose
this functionality to minio manta gateway.
Manta is an Object Storage by [Joyent](https://www.joyent.com/)
This PR adds initial support for Manta. It is intended as non-production
ready so that feedback can be obtained.
With https://github.com/minio/minio/pull/4869 maximum size of a single
multipart upload part in not restricted to 100MB. 1TB maximum object
size limitation is no longer applicable too.
Paul Stack